I have read that infinitives in English can function as one of three parts of speech, namely nouns, adjectives and adverbs. So for example in the sentence:

I am going to buy groceries.

Do we assign the preposition "to" to the infinitive form of "buy" as in "to buy" or is that preposition part of the preceding verb as in "going to" and then "buy" is just the bare infinitive? Furthermore, what part of speech would "to buy" be if the former is correct? I would assume an adverb, but not sure.

3 Answers 3


I am going [to buy groceries].

Infinitives don't function as nouns or any other part of speech.

Their part of speech is verb and their characteristic function is that of head (or predicator) in a verb phrase.

The "to" component, if present, is not part of the verb but a separate element, a subordinator. Thus, in your example the infinitival verb is "buy", which is functioning as head (predicator) of the bracketed infinitival clause, and "to" is a subordinator functioning as a marker of the non-finite verb phrase.

  • What you say is completely different than everything I read. Can you provide some reference for your answer? Commented Jan 4, 2023 at 6:24
  • That is because most grammar books for English learners are based on high school grammar from the 1950s and earlier. Imagine if your university science books were all based on the science lessons that someone remembered from 70 years ago, and didn't include any modern discoveries. This is a fairly conventional modern analysis of grammar which you could read about in, for example, the Cambridge grammar of English Language.
    – James K
    Commented Feb 3 at 20:08

In English, this is one way to express the future with the present progressive of the verb go. It is also, just the present progressive of the verb:

  • I'm going to buy groceries later [future idea]. Compare:
  • I'm going to leave now. [as opposed to I go etc.]

Parse: I'm going/to buy/groceries/now. [now, not future: it is what I'm doing now.]

to buy is just an infinitive. For the future sense, going to is followed by a bare verb. Although people call this the future with going to, that does not mean that grammatically going to is a thing. It's just descriptive. It's present progressive + yo-infinitive + complement. And it can mean doing something in the future or right now depending on the context.

Where you get to + an infinitive, that is called a to-infinitive.

  • We decided to leave early.

The sentence "I am going to buy groceries" has two slightly different meanings. Most often it means "I will buy groceries". However it can also mean "I am going in order to buy groceries".

In the first use I would regard "going to" as a unit and "buy" as a bare infinitive, though I think that in practice "am going to buy" should be thought of as a particular form of the verb "to buy".

In the second use I would regard "to buy" as the infinitive. The adverb would be the complete phrase "to buy groceries".

  • In the first case would "going to" be considered a modal verb? Commented Jan 3, 2023 at 14:59
  • I'd say that the "to" is an infinitival marker in both cases.
    – BillJ
    Commented Jan 3, 2023 at 17:23

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