Reopen note:

There is a quite finite and modest amount of evidence in the literature about this issue, which members can record here as they see fit. Less than there is for example about what a noun is or what a verb is, or how to tell what the Subject of a sentence is. It's a simple question, that people need to be able to get answered on a site such as this. It is of continuing interest as more recent questions such as this attest to. The fact that people disagree about the outcome of the evidence does not mean that there is too much evidence to be documented. Quite the reverse. They disagree because there is relatively little evidence altogether. However, it is seriously interesting evidence.

The question

What part of speech is to as in:

  • I need to know.
  • To err is human, to forgive divine.
  • What am I to do?

This question is not really about the difference in meaning between the examples. It is a question about what grammatical or syntactic reasons we have to classify this item as a particular part of speech. If arguing that it is 'its own part of speech', I'd like to know what grammatical - but not historical - reasons you have for your claim.

Just to clarify, I'm not asking about what assignment any particular authority or publication gives it. I'm interested to know the reasons why. Your own personal insights are also very welcome.

If you're a grammar anorak - like me - then you may want a list of the contenders out there from other grammarians. In no particular order:

  • preposition
  • subordinator
  • non-finite auxiliary verb
  • modal verb
  • inflectional element

Can you help?

  • 1
    The to is sui generis. We can call it a particle, as Wikipedia does, or a subordinator, as Pullum does, or use a phantasy word invented for the occasion. (And no, it is not a verb, and no longer a preposition, as both Pullum and the top answer to the linked question explain.)
    – RegDwigнt
    May 30, 2014 at 1:03
  • 6
    It's a Complementizer in particular, it's part of the for...to infinitive complementizer, which marks infinitive clauses. For marks the subject of the infinitive clause, to marks the verb (which is, of course an infinitive verb form). May 30, 2014 at 1:31
  • 2
    I voted to reopen this thread. :) -- I agree with you in that the other thread didn't address the meat of your question. It sounds like you are looking for a discussion on the syntax of the "to" infinitival w.r.t. its "to". I don't see why such a discussion wouldn't be allowed to be held among those that might be interested in having it. Perhaps you could change your title to reflect more accurately the type of discussion that you are looking for.
    – F.E.
    May 30, 2014 at 2:11
  • 1
    I'd like to see this question reopened, if only because it seems the OP has already reflected on this point and would like to hear what others believe. One could argue that is a POB question though, and no "right" or "best" answer exists.
    – Mari-Lou A
    May 30, 2014 at 3:16
  • 2
    To start with, in normal speech almost all such words (articles, auxiliaries, complementizers, prepositions, pronouns) are unstressed, so in practice one hears some syllable with /ə/ in it. There's very little processing involved, since there's a bump in the speech stream where the listener is expecting a bump. Think of them as color-coding, syntactic arrays to take the place of the missing inflectional arrays. That's what they are. As for "Evidence", that depends on your presuppositions; if you're looking for something, you're likely to find it. May 31, 2014 at 20:36

2 Answers 2


You can rather easily argue that to belongs to its own part of speech. The only evidence you need is that it does not fit into any of the major part of speech categories. You will just construct grammatical tests showing that it is not a noun, not a verb, not a preposition, etc. Following examples suggest that to is not a verb or noun, and that other prepositions cannot precede verbs in their base form (respectively).

*Nobody shall to.
*The to is ready.
*It is hard to see with run.

Of course, you can go on with this type of argument with pretty much any high frequency word until you have so many parts of speech that the concept is no longer useful. Of possible interest is Croft's discussion of Word Classes in Radical Construction Grammar.

  • I kind of agree. But, that's only if your test shows that it does not fit, and only if your willing to eject from your categories other items on the same basis if they fail comparable tests. I'm not convinced about your actual tests though. The first in particular seems analogous to Nobody shall can which would rule out can from being a verb. So there still seems to be scope for to being a verb. If S-conjunctions are prepositions then we have: Because try as she might regarding the preposition claim. You haven't given any tests for other categories. Can you do so? Jun 1, 2014 at 7:18

One option that doesn't appear on your list is that it isn't even a word. Perhaps it's merely an open form bound morpheme.

I do see some evidence against its being an affix. If it is an affix, it's the only affix I know with an open form. Other affixes have closed or hyphenated forms. If it is an affix, it's the only affix I know that is occasionally suppressed. For example, except for the to, "I'll help you to do it" and "I'll help you do it" are identical in grammar and meaning.

I also see some evidence for its being an affix. It only appears at the beginning of an infinitive. The to marks the infinitive form of a verb in much the same way as the -ing marks the gerund. The infinitive form is created in other languages by using affixes.

It seems much more likely to me that the infinitive marker is a slightly odd example of inflectional morphemes than that it is the sole example of an entire category of grammatical word function. That may be slim and subjective, but I count that as evidence against its being a full-fledged word.

  • I like the idea. I'm wondering about items that come in between to and the plain form of the verb: to not comply or to bravely go. Do you think this poses a bit of a problem for the clitic/affix proposal? Nov 28, 2014 at 11:01
  • Yes, it does pose a problem. One solution (which I do not endorse) is to declare that the split infinitive is an error. Another is to claim that those modifiers are, in fact, inserted inside the one-word open form infinitive, which is something we abso-bloody-lutely do in other circumstances, even without the invitation that the open form suggests. Nov 28, 2014 at 15:07
  • Come to think of it, it's not the only affix I know that's sometimes suppressed. The commands "Move quickly" and "Move quick" are the same. We drop the -ly without changing either the grammar or the semantics. May 19, 2015 at 22:58

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