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I was reading an article about the use of "why" as an adverb. I thought about what other function the word can have and came to the reasoning that it can be a conjunction joining clauses. I looked up a number of dictionaries:

Merriam-Webster Dictionary(yes, conjunction)
why
conjunction
2 : for which : on account of which
know the reason why you did it

American-Heritage Dictionary (yes, conjunction)
why
conj. 1. The reason, cause, or purpose for which:
I know why you left.

Random House Kernerman Webster's Dictionary (yes, conjunction)
conj.
2. for what cause or reason: I don't know why he left.
3. for which; on account of which (usu. after reason to introduce a relative clause): the reason why she refused to go.
4. the reason for which: That is why he returned.

Cambridge Dictionary (yes, conjunction)
conjunction, adverb [not gradable]
for what reason:
She’ll ask why you don’t have your homework.

Oxford Living Dictionaries (conjunction not mentioned, listed as "relative adverb")
relative adverb

1(with reference to a reason) on account of which; for which.
‘the reason why flu jabs need repeating every year is that the virus changes’
1.1 The reason for which.
‘each has faced similar hardships, and perhaps that is why they are friends’

Collins Dictionary (doesn't mention conjunction, lists is as "pronoun")
pron
for or because of which: there is no reason why he shouldn't come.

Yet on the Collins Learner's Dictionary it lists it both as pronoun and conjunction
2. conjunction You use why at the beginning of a clause in which you talk about the reasons for something.

  • He still could not throw any further light on why the elevator could have become jammed.

  • Experts wonder why the U.S. government is not taking similarly strong actions against AIDS in this country.

  • I can't understand why they don't want us.

3.pronoun You use why to introduce a relative clause after the word 'reason'.

  • There's a reason why women don't read this stuff; it's not funny.
  • Unless you're ill, there's no reason why you can't get those 15 minutes of walking in daily.

So in all examples where Collins lists it as a pronoun we have "why" coming directly after the word "reason". Oxford Living Dictionaries doesn't list it as a pronoun, but a "relative adverb".

I didn't find "why" anywhere in the Wikipedia Conjunction article, but did find it in the Wikipedia Conjunctive adverb article.

I'm wondering why in these two particular dictionaries (Collins and Oxford Living Dictionaries) we don't find "why" listed as conjunctions? Are they not viewed as such in modern grammar?

Also, I've been warned in the past not to place my trust in dictionaries when recognising word categories or functions, which is another reason why I'm asking this.

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    Are other question words considered some kind of conjunctions? I'd trust a common thread among published dictionaries over Wikipedia.
    – Mitch
    Nov 27, 2018 at 3:11
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    Why is a just a rather odd wh-word. Its distribution is very limited -- it can only have the word reason as its antecedent, and since it's never the subject it's always deletable. Consequently it behaves strangely, as you and others point out. Nov 27, 2018 at 3:39
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    @Zebrafish - I think you're asking the wrong question. Why is a wh-word, and they have their own category. They sometimes act like conjunctions do, but so do many other words. Asking about the Part of Speech of some grammatical word is a waste of time, because the Parts of Speech were designed for describing inflected languages, and English is uninflected, which means there are a lot of little words lying around that don't have meanings or general purposes, but rather fit specifically in some construction to make it work. Wh-words are one such group of words. Nov 27, 2018 at 17:41
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    @Kris "Searching for 'Is why a conjunction?' is not the end of research." I don't remember ever saying that one string search was the end of my research, nor does my question indicate that. My question contains 7 links to dictionary entries, including a learner's dictionary, and two Wikipedia articles. I also don't remember claiming that a word should have only one function "cast in stone". I asked this question with reference to the fact that two respected dictionaries did not list "why" as a conjunction, but rather as a "pronoun" and "relative adverb". I don't know why this is.
    – Zebrafish
    Nov 28, 2018 at 9:31
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    The difference between an adverb and a subordinating conjunction is not very substantial. Most subordinating conjunctions introduce adverbial clauses and many if not most can also be used as adverbs with the same sense, without a clause (but often presupposing one). That's the issue here -- POS are important in a Big Bag of Words kind of grammar, where all you see is words and tags on them. But that's not how English works -- English is all phrases and clauses acting as adverbs or nouns or adjectives or verbs -- so POS gets confused between the clause and its head word. Nov 28, 2018 at 16:03

5 Answers 5

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The word why can obviously be used as a conjunction, as your examples clearly indicate.

The problem is with the dictionaries.

To see why, consider this Wikipedia article on Natural Language Processing. In the original “rules based” approach to language processing, the analysis of a speech fragment depended on the fragment conforming to an expression that could be generated from a grammar. The association of a given word with a part of speech would therefore determine the expressions that could be generated.

However, the “rules based” approach has been largely superseded by the “statistical” approach, partly because the rules themselves became complex and unwieldy.

But my real point is this: Dictionaries are written by human beings with a fundamentaly limited rules-based approach to language processing. As the human user attempts to apply the dictionary rules (parts of speech, etc.), he or she inevitably runs up against the same problems as the computer programmer trying to implement similar rules in their code. The rules printed in the dictionary are simply not powerful enough to deal with the range of expressions found in real life.

From the statistical point of view, why is found in multiple roles. However, the dictionary writer, aiming at readability instead of completeness, inevitably has to leave some of these out.

It is worth noting that even a simple natural language processing program like Siri (or the Office Online spelling assistant) has already analyzed more sentences than you or I could read in a thousand lifetimes. Their “dictionaries” and “grammar manuals” go far beyond what can be found in Collins or the Oxford.

So in practice, the occurrence of why in the role of a conjunction means that the part-of-speech attribute conjunction can be applied to the word why.

I suppose we could go further and argue about whether the assignment of an attribute means that something actually “is” something, with a digression into deep epistimology, social constructions of meaning, and that kind of thing. However, if we accept that the statistical concept of natural language processing has been validated through its practical application, even for the sake of argument, then we should be willing to accept the assignment of attributes by the programs that have analyzed the largest number of examples, your own examples counting among them.

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    Your argument seems to be that with a large enough corpus any word can be found in any grammatical role, so parts of speech have no meaning?
    – Stuart F
    Mar 29 at 15:37
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Why is a just a rather odd wh-word. Its distribution is very limited -- it can only have the word reason as its antecedent, and since it's never the subject it's always deletable. Consequently it behaves strangely, as you and others point out.

............

I think you're asking the wrong question, Zebrafish. Why is a wh-word, and they have their own category. They sometimes act like conjunctions do, but so do many other words.

Asking about the Part of Speech of some grammatical word is a waste of time, because the Parts of Speech were designed for describing inflected languages, and English is uninflected, which means there are a lot of little words lying around that don't have meanings or general purposes, but rather fit specifically in some construction to make it work. Wh-words are one such group of words.

............

The difference between an adverb and a subordinating conjunction is not very substantial. Most subordinating conjunctions introduce adverbial clauses and many if not most can also be used as adverbs with the same sense, without a clause (but often presupposing one). That's the issue here -- POS are important in a Big Bag of Words kind of grammar, where all you see is words and tags on them. But that's not how English works -- English is all phrases and clauses acting as adverbs or nouns or adjectives or verbs -- so POS gets confused between the clause and its head word.                                                                                                     J Lawler: comments preserved

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    Thank you for posting John Lawler's comments as an answer!
    – Mori
    Apr 6 at 12:14
  • I don't think he'd disapprove. Apr 6 at 14:57
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Superficial Answer

It all depends on how a conjunction is defined. Now, when discussing whether why is a conjunction, we're talking about a subordinate conjunction.

If a subordinate conjunction is defined as anything and everything that introduces a subordinate clause, why could indeed be a subordinate conjunction because it does introduce a subordinate clause.

In I wonder why you left, for example, why could be "loosely" analyzed as introducing the subordinate clause why you left.

Real Answer

I've used the word "loosely" above to show the suspicion at the back of my mind that this kind of definition might be too loose to be useful. First, it's not clear whether it's why that introduces the subordinate clause why you left in the above example. Apparently, the same why can be used to introduce a main clause as in Why did you leave?, where no dictionaries would class this why as a subordinate conjunction.

Second, if we were to class why as a subordinate conjunction, should we also class other interrogative words (who, what, which, when, where, and how) as subordinate conjunctions when they're used to "introduce" a subordinate clause as follows? I wonder who she is. I wonder what she does. I wonder which it is. I wonder when she leaves. I wonder where she is. I wonder how she is.

I know of no dictionary or grammar that classes who, what, and which as subordinate conjunctions. Those dictionaries that class why as a subordinate conjunction also class when, where, and how as subordinate conjunctions, but even they don't class who, what, and which as subordinate conjunctions.

Of course, as long as they "loosely" define a subordinate conjunction, they can class anything and everything that "introduces" a subordinate clause as a subordinate conjunction. But I seriously doubt that that's the right way.

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I would add the following to the OP's observation and J. Lawler's thoughts:

Reason is not unique. It appears that synonyms and related words, such as explanation, rationale, excuse, justification, motive, argument, and grounds are among other possible antecedents.

Yet when the actor does what is uncommon and not to be expected, we then ask for an explanation why he acts as he does.
Jens Kulenkampff; "How to Improve On Bittner's Proposal" in Marco Iorio and Ralf Stoecker (eds.); Actions, Reason, and Reasons (2015)

The other, in requesting an explanation why the same performer chose this particular occasion to express the belief that p, asks the point and purpose of this particular speech act.
Antony Flew; Atheistic Humanism (2011)

But if it were legitimate to demand an explanation why God has the essence he would have if his essence were different, then it would be legitimate to demand an explanation of the fact that God has the essence that he actually does.
Martin Lin; "Metaphysical Rationalism" in Jack Stetter and ‎Charles Ramond (eds.); Spinoza in Twenty-First-Century American and French Philosophy (2019)

Every couple of weeks that I came down and stood in line, I would ask myself for an explanation why.
S. C. Graham; Quarter Century Crisis (2014)


This is the primary rationale why intense exercise should be avoided during humid conditions and/or additional precautions should be employed to supervise athletes training or performing in hot/humid environments.
Ben Kennedy; Why is America so Fat? (2004)

Additionally, although like Smith, Sraffa sees the net product excluding means of production, unlike Smith he sees it also excluding subsistence wage good, without providing any rationale why this should be the case.
Howard Nicholas; Explorations in Marx's Theory of Price (2023)


If Christ had not stood and knocked, your sin had been less; there had been some excuse why you did not admit him.
The Practical Works of David Clarkson, Vol.II (1865)

There is no justifiable excuse why the president should have such enormous and obscene powers as to be the sole controller of the nation's vast oil resources.
Uche Nwakudu; How to Fix Nigeria (2011)


There was no conceivable worldly motive why they should either receive this doctrine themselves or preach it to others.
James Copland; Reasons why we believe the Bible (1878)

It furnishes a possible and probable motive why he should not wish to disturb her in the enjoyment of Hesdra's property, and a reason why the court should admit the will to probate notwithstanding his reprehensible conduct.
The Northeastern Reporter, Vol.23, "In re Hesdra's Will" (1890)


He argues that this question is raised for purposes of a rational justification why we must be bothered with the issue of the moral way of life inasmuch as, to Beauchamp, the demand to live a moral life could stand in conflict with out own self-interest or it could also stand in conflict with our other commitments.
E. Babor; Ethics (2006)

For example a book might give a rational justification why you should act in a courageous manner but then fail to tell you how to actually become courageous.
Darin Penzera; Heroic Egoism (2012)


To speak of confirmation, we need a clear argument why the confirming evidence increases the probability of the viability of H.
Richard Dawid; "The Significance of Non-Empirical Confirmation in Fundamental Physics" in Radin Dardashti, ‎Richard Dawid et al. (eds); Why Trust A Theory? (2019)

If he sat on this side of the House he would find it equally easy to find every argument why hon. Gentlemen here are always right and hon. Gentlemen there are always wrong...
J.D. Rees; Parliamentary Debates (1912)


In his classic statement about The Two Cultures, C.P. Snow is able to justify with ease why humanists should study the sciences but struggles to articulate grounds why scientists should study the humanities.
Patrick Deneen; Why Liberalism Failed (2019)

There are no principle grounds why the state ought to inflict criminal sanctions.
Alon Harel; Why Law Matters (2014)


After posting the above, I found this definition in the OED online:

why (adverb):

II.3.a. On account of which, because of which, for which. Usually, and now almost always, after reason (formerly also after other, broadly synonymous, nouns). Also with ellipsis of elements of the subordinate clause.

I question the accuracy of the formerly label.

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  • How is this an answer?
    – JK2
    Apr 7 at 4:13
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    @JK2 ELU doesn't have a contribution that lies between a comment and an answer-- a kind of "coda" to another answer. This is meant as a correction to the observations made by both the OP and the late J. Lawler, the latter posted as an answer. I will label it as such. It required documentation that goes beyond the length of a comment and was not intended as a full answer.
    – DjinTonic
    Apr 7 at 7:22
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In the following sentence, why might seem to be a subordinating conjunction, which simply joins two clauses:

I don't know why he got angry.

But according to the Oxford Languages, it's an interrogative adverb, which here means for what reason.

Now, consider the following:

I don't know the reason why he got angry.

In this sentence, why (= for which) is a relative adverb, and why he got angry is a relative/an adjective clause modifying the noun reason. Where, when, and why modify the verb, so they are relative adverbs, not relative pronouns:

  • That was the place. + We ate in that place. = That was (the place) where we ate. "In that place" (= there) is an adverb giving information about where it happened.
  • That was the time. + We met at that time. = That was (the time) when we met. "At that time" (= then) is an adverb giving information about when it happened.
  • That was the reason. + He got angry for that reason. = That was (the reason) why he got angry. "For that reason" (= therefore) is an adverb giving information about why it happened.

See also

  • Complete English Grammar Rules: relative adverbs
  • John Eastwood, Oxford Learner's Grammar - Grammar Finder, entry 271: relative adverbs
  • Bas Aarts, Oxford Modern English Grammar, p. 197: wh relative clauses
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    There are ongoing debates about the classification of words occupying unconventional slots as the result of deletions of various kinds, and associated usages (intransitive prepositions, for instance). John Lawler classes why here as a relative pronoun. Dec 24, 2023 at 14:04
  • I don't know what Swan's logic is but surely it's incorrect to call it an informal version of "I don't know the reason why he got angry." We find in Shakespeare's All's Well That Ends Well "But never hope to know why I should marry her." And he also uses "ask why". So if it's an informal/non-standard form, it's one that was in Shakespeare and has persisted over 400 years; at what point is such a thing elevated to correct grammar?
    – Stuart F
    Dec 24, 2023 at 14:16
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    @EdwinAshworth: "it's a pronoun that can only refer to one word: reason" A noun and pronoun that are used together!? As far as I know, a pronoun replaces a noun to avoid repetitiveness.
    – Mori
    Dec 24, 2023 at 14:56
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    @Mori Your two examples are not the same construction. They mean the same thing merely because the verb know can take an interrogative clause or an NP. Cf. I wonder why he got angry vs. *I wonder the reason why he got angry.
    – JK2
    Mar 31 at 4:44
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    @Mori Traditional grammar and many, if not all, modern grammars class the interrogative word why as an adverb. If it's an adverb, it can't be a conjunction at the same time.
    – JK2
    Apr 4 at 3:43

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