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I'm motivated to ask this question because of this other question on ELL SE.

It seems to me that that's a prepositional phrase, but I don't see 'once' in any lists of prepositions, and the only dictionary I found listing it as a preposition is Simple Wiktionary.

So, consider this sentence with 'once':

Once I get home, I get hungry.

and compare to this sentence which definitely has a prepositional phrase:

After I get home, I get hungry.

Is 'once' a preposition in that sentence? Is it ever?

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    I would like to note that when is not a preposition either, yet it could be substituted for both once and after. If Simple Wiktionary is the only dictionary you found listing it as a preposition, I would ignore it.
    – Jake Regier
    Commented Jul 16, 2015 at 3:24
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    @JakeRegier But every self respecting modern academic grammar says that it's a preposition. Not a good idea to try to get your part of speech info from dictionaries. :)
    – Araucaria - Him
    Commented Jul 16, 2015 at 10:24
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    @Araucaria Source? If you have one, it might help to answer the OP's question. Otherwise, I have no idea who these "self respecting academic grammar[s]" are.
    – Jake Regier
    Commented Jul 16, 2015 at 12:26
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    @Araucaria Yes, but there are two problems with that: 1) Both OP and I speak AE and those are BE texts, which could make a difference; and 2) You still don't cite anything relevant from those texts. And, well, whether or not these grammarians respect themselves is beside the point.
    – Jake Regier
    Commented Jul 16, 2015 at 13:06
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    @JakeRegier No,it makes no difference. when is the same part of speech just like verbs are still verbs etc. The Cambridge Grammar of the English language is not a BE text. Many of the authors are Am also Australian. It is regarded by many as the best grammar of the English language ever written. Erm, dictionaries are not. I don't particularly feel the need to explain prepositions. I was just responding to your comment where you tried to "correct" the OP.
    – Araucaria - Him
    Commented Jul 16, 2015 at 14:45

3 Answers 3

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Both once and after are being used as subordinate conjunctions in your sentences. Once I get home and After I get home are fragments if the sentence that follows is missing.

I admittedly am not the greatest grammarian here, but I cannot think of a sentence using the word once where it doesn't clearly fit a different part of speech.

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  • Are you saying that 'after' is not acting as a preposition in my example sentence? That's pretty much the canonical example from school. If that's not a preposition, then I don't know what is.
    – DCShannon
    Commented Jul 17, 2015 at 19:48
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    After is definitely a preposition, but its function is as a subordinating conjunction in your example sentence. The same goes for the term once; however, anytime the word once is used it will serve as an exact part of speech (adjective, adverb, noun, etc.) in whichever sentence.
    – Dunnup
    Commented Jul 18, 2015 at 20:10
  • Alright, I looked up subordinating conjunction, and that looks correct. I don't think any clear distinction was made between subordinating conjunctions and prepositions when I was in school. To identify prepositions we were told to look for patterns, and this pattern was one of them.
    – DCShannon
    Commented Jul 20, 2015 at 17:27
  • +1, I wrote a better answer to the ELL question after reading your answer here. Still not marking it as accepted, as there could be more of an explanation of the "Is it ever?" part of the question.
    – DCShannon
    Commented Jul 20, 2015 at 17:41
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    As Dunnup says, after is often used as a praeposition, as in after school, I got drunk. It only has a only noun following it (school), which is typical of praepositions. In after I left school, I got drunk, it introduces a (subordinate) clause, which you can recognise by the verb (left). In that case, it is not a praeposition but a (subordinating) conjunction. Once is never used with only a noun, but always as either an adverb or a subordinating conjunction.
    – Cerberus - Reinstate Monica
    Commented Jun 10, 2018 at 15:09
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Once (after or when) I get home, I get hungry: In any case, all three of these words are acting to join the two clauses, but it is a bit easier to see the functions of these words when the sentence is in standard order (S+V+C):

I get hungry after (or once or when) I get home.

It should be pretty clear that these all function as adverbial conjunctions (aka 'conjunctive adverb' or 'subordinating conjunction') to join the two clauses with a time relation. Words are just words, most of the time they can't be typed until they are used in a phrase or sentence. Accordingly, prepositions are always prepositions, but some words like 'after', 'when' and 'once' can have different functions (types), depending on how they are used.

The hard rule about prepositions is that they are always followed by nouns. But 'once' never functions as a preposition, and 'after' in this case, isn't one either, even though it is followed by a noun. Prepositions never bridge clauses, they are always internal. Another rule is that they establish a meaningful relation between the noun that immediately precedes them (or a preceding verb phrase in their clause), and their object noun. In this sentence all three words create an adverbial relation between the clauses, none of them are functioning as a preposition. They establish a time relation (sequence) between the clauses, that's all. They are not adverbs because the have only an indirect relation to the verbs in either clause.

In this sentence, 'once' an adverbial conjunction.

Update 01/2019: Maybe once can function as a preposition.

How about this sentence.

I talk to my sister once a week. S+V+AC(pp)+AC(pp)

Once seems to be working as a preposition in this case. It is creating a relation between the verb and its object a week. It definitely has an object and that relation determines the meaning of the sentence. This use requires the present simple - We can't say, * I talk to my sister once, it would have to be in another tense to be an adverb and have the meaning one time.

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  • This answer just expands on what Dunnup has already said in his answer. I hope my explication is also useful.
    – Ubu English
    Commented Jun 14, 2018 at 8:57
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Merriam-Webster 3rd international calls it a conjunction, but this is not really a linguistic analysis, just a convenient label. It also labels the temporal 'after' as a conjunction. I wouldn't say preposition: prepositions generally need an object. So in sentences such as 'He's always chasing after some girl.' 'after' is a preposition.

This is actually a very technical subject. Maybe John Lawler will weigh in on how to tell a preposition.

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    There are lots of prepositions that don't have an object (give up, get out, hang on, run around, etc.), but in the example given, the following clause “I get home” is the object.
    – Janus Bahs Jacquet
    Commented May 15, 2017 at 18:17
  • @JanusBahsJacquet: I'm not sure I'd call those praepositions, though. I'd rather say they're praepositions used adverbially, or just adverbs, or maybe even particles?
    – Cerberus - Reinstate Monica
    Commented Jun 10, 2018 at 15:12
  • @Cerberus Different analyses yield different results, of course; but I’ve been convinced by CaGEL that they are in fact prepositions, whether they have an object or not.
    – Janus Bahs Jacquet
    Commented Jun 10, 2018 at 15:21
  • @JanusBahsJacquet: If our goal is then to group both uses of around (to run around, to run around the house) together, where one use is not 'prae' at all, then might the word praeposition not be confusing to some people? Would it not be better to use a different (new?) word for this group? Then praeposition could be reserved for its conventional use. (Whenever I have a problem with CGEL, it is seldom about analysis, but almost always about terminology, which I think distracts from the greatness of this work.)
    – Cerberus - Reinstate Monica
    Commented Jun 10, 2018 at 15:24
  • @Cerberus Yes, I agree‑preposition is a bad term all around and always has been—what I agree with CaGEL about is that it remains the same word class/POS, whatever we want to call it. Some prepositions (with objects) aren’t ‘pre’ either, but postpositions (‘through the night’ vs ‘all night through’). Luckily, I don’t think most people realise what preposition really means; if they do, they’re probably smart enough to realise also that it’s not necessarily a very useful descriptor.
    – Janus Bahs Jacquet
    Commented Jun 10, 2018 at 16:07

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