What is the easiest way to understand what an infinitive is?
How do I know which verb in which sentence is an infinitive?
For example, let us take this website:
This is the example I am interested in:

  1. I need to run every day. (The infinitive form with the word to is called the full infinitive or to-infinitive.)
  2. I must run every day. (After certain verbs, the to is dropped (more on this below).)
  3. I run every day. (This is not in the infinitive form. This is a finite verb, i.e., a verb functioning as the main verb.)

How do I differentiate between example 2 and example 3?
How do I know 3 is not an infinitive but 2 is ?
If 2 had been I must run to school every day would that make it a non-infinitive?

This is probably a completely different question, but I will put it here anyway.
When comparing with other languages, if it says infinitive, should I consider it to have the same meaning as the English infinitive, or can it have a different meaning, according to the language?

Let us take the example of Korean. This Wikipedia page has an entry for infinitives in Korean. If I understood correctly, "infinitive" is the form of the verb that is unchanged according to tense, voice or anything similar. (So watch may be an infinitive but watched is not). In Korean, the unchanged form of a verb is called root (example, watch = 보다). This root changes form (prefixes, suffixes etc, based on negation, formality, tense, etc). Can this root be called an infinitive as well? (in my opinion, probably not, as the explanations in the wikipedia page I linked seems to use root and infinitive as different entities, but how do I explain the difference)?

  • Substitute I with he. "He must run every day" vs. "He runs every day". In the second case, you had to inflect the verb, thus it can't be the infinitive.
    – RegDwight
    May 29, 2014 at 12:57
  • so in case 3, when changing the sentence from first to second person, the verb has to change it's form, so it is not an infinitive. Is this how I should interprete it?
    – user17915
    May 29, 2014 at 13:01
  • Interestingly, Latin infinitives have voice and tense/aspect marking: laudare (oresent active), laudari (present passive), laudavisse (perfect active), laudatus esse (perfect passive), laudaturus esse (future active) and laudatum iri (future passive). May 1, 2020 at 16:32
  • in this case simple: the first two sentences use 'run' with an auxiliary verb, the third doesn't. Jul 20, 2021 at 2:56

2 Answers 2


To answer the last question first, Infinitive is a term from Latin grammar.
It refers in Latin to one of several tenseless verb forms.

Etymologically, infinitive means 'unending' (just like infinite); here the reference is to lack of tense.
In other languages, infinitive is often used to describe one, or more, untensed verb forms.
The Latin terms Participle, Gerund, and Gerundive are also in use for untensed verb forms.
What these terms mean, in each language, has to be determined individually. There is no consistency.

The rest of the question is about English infinitives, and here the trick is knowing
that there are two different meanings for infinitive in English grammar.
If you keep them straight, you won't be easily confused.

  1. Infinitive can refer to the infinitive form of the verb, e.g, be, run, go, see, feel, have, admit, ask
    Every verb in English (except defective verbs like born and modal auxiliaries) has
  • Three Principal Parts: be, was, been; run, ran, run; go, went, gone; see, saw, seen;
    feel, felt, felt; have, had, had; admit, admitted, admitted; ask, asked, asked.
    These three verb forms are called, in order, the Infinitive, the Past, and the Past Participle.
  1. Infinitive can also refer to infinitive clauses of various kinds, which are subordinate clauses
    that use infinitive verb forms, instead of present, past, or participial forms.
  • For him to err is human ~ He wants to leave early ~ She's the one to see about that.
    These are normally marked with the for...to complementizer, though the for that
    marks the subject of the clause is frequently absent, along with the subject itself.

As @YellowSky has pointed out, most infinitive clauses are governed by the matrix predicate.
There are a number of auxiliaries that must be followed in construction by an infinitive verb form.

All the modal auxiliary verbs (including the NPI semimodals need and dare,
as well as the periphrastic modals like hafta, wanna, oughta, able to, and need to)
must be followed immediately by an infinitive verb form, and the same is true of do
in its Do-Support and Emphatic senses.

  • Do you like ice cream? but not *Do you liking/liked ice cream?
  • I did take it out! but not *I did taking/took it out.

These infinitive verb forms are part of a verb phrase and don't use the complementizer to, because they don't introduce complements, just the next verb in a verb chain. See the Verb Phrase Guide for details.

Executive Summary:

Infinitive clauses are very complex, and they use infinitive verb forms.
Infinitive verb forms are used in infinitive clauses (usually marked by to);
infinitive verb forms also occur with auxiliary verbs in verb phrase constructions
(usually without to).

  • Isn't born 1. the Past Participle of the verb bear, bore, born; 2. adjective 'existing as a result of birth'?
    – Yellow Sky
    May 29, 2014 at 17:55
  • Yes, both. But when not referring to the mother, it's always passive in form; i.e, there is no English verb to indicate birth circumstances except be born (unlike Spanish nació, an active verb form); this is rather like the fact that there is no English verb meaning be named (like German heisst, an active verb form). In both cases, the construction is effectively defective.
    – jlawler
    May 29, 2014 at 18:02
  • Well, the child is born (by her mother) - naturally, it's passive. But the mother bore children - naturally, it's active.
    – Yellow Sky
    May 29, 2014 at 18:14
  • 1
    In that case, one uses the spelling is borne (by). This is a true passive, with a direct object promoted to subject and an agent subject demoted to chômeur status. Born without the E is only used in the deponent construction, without agent.
    – jlawler
    May 29, 2014 at 18:28
  • 1
    Sounds like Chomsky; it's easy to see why - infinitive, first auxiliary, inverts, contracts. But it inflects, so it's not a total match. Plus it's conditioned by syntax and has no meaning, whereas modals are overstuffed with meaning.
    – jlawler
    May 1, 2020 at 2:57

Infinitives are rather different in different languages, and some languages, like Bulgarian, don't have infinitives at all. In some languages infinitives have a special suffix and are easily recognized, e. g. in Russian the most typical infinitival suffix is -ть, like in писать 'to write', читать 'to read', etc.

In English the infinitives are in most cases marked by the infinitival particle 'to' (to-infinitive), but sometimes they are not (bare infinitive), still the list of cases when they are not mark is pretty short. The English unmarked infinitives follow another verb. Here is the list of the verbs that are always followed by an unmarked infinitive, infinitives are marked in bold:

  1. The nine defective verbs: can, could, shall, should, will, would, may, might, must: I cannot do that.
  2. The verb 'let': Let me help you.
  3. The verb 'help': He helps me do my homework. (Nowadays many use the to-infinitive after 'help'.)
  4. The verb 'make' in the meaning 'to cause': He made me cry.
  5. The verb 'do' (Past 'did') used to form questions, negative sentences, and emphatic sentences in the Present Simple Tense and in the Past Simple Tense: Do you know him? (question), I don't know him. (negative), I do know him! (emphatic);
  6. After 'needn't', 'need not' and questions with 'need [subject]': You needn't go there.

Unlike many languages, the English infinitive has different forms, 4 for an intransitive verb (they don't have Passive forms), and 8 for a transitive one. English infinitives can change for voice (Active: to write, Passive: to be written), for aspect (Non-Continuous: to write, Continuous: to be writing), for perfectness (Non-Perfect: to write, Perfect: to have written), and there are Perfect-Continuous infinitives: to have been writing. So, you got it wrong, the infinitives can change after the grammar categories.

  • 1
    I must say I find it rather odd that #3 implies help to [verb] is more likely "nowadays". I've no idea what the actual usage and trends have been over time for that specific verb, but it seems to me to was once more common in #4 ("He maketh me to lie down in green pastures", and all that). That's to say, if there'e a "general trend", I think it's towards discarding to in more and more contexts. Btw - there's also "He gave me to understand", but I maybe that's not quite the same construction. May 29, 2014 at 14:08
  • 1
    #6 is not quite right. It would be more accurate to say unmarked infinitives follow the verb need outside affirmative declaratory contexts when do-support is not employed – or in layman’s terms, after needn’t, need not and questions with need [subject]. Also supercat is right: you say transitive verbs have 6 infinitival forms, which should be 8 (the continuous passives are left out). Jul 19, 2021 at 21:17
  • 3
    @YellowSky That is absolutely not true. They are rare, but they are used when the situation calls for it. An example of the former can be found in one of the Harry Potter books where “he didn’t have a clue what was going on, but he didn’t seem to be being expelled”; and I am quite sure I read an example of the latter just a few days ago in an article, something along the lines of “we have argued most of the literature generally assumed to have been being written under Jón’s stewardship was in fact written later”. Jul 19, 2021 at 22:19
  • 1
    Re: "The only tendency about infinitives I know of is the increasing usage of to-infinitives after 'help'." - this is not what numerous corpus studies have shown. There are tons of research on this, you could start with Callies 2013 Bare infinitival complements in Present-Day English doi.org/10.1017/CBO9781139060998.011 or Lohmann 2011 Help vs help to: a multifactorial, mixed-effects account of infinitive marker omission doi.org/10.1017/S1360674311000141 @FumbleFingers you might find this interesting too
    – Alex B.
    Jul 20, 2021 at 16:36
  • 1
    @AlexB.: Thanks for the links. It's just taken me 10 minutes to get back into the swing of thinking about this one, and all I'm "reasonably confident" about so far is that I still think there's something wrong with talking about the increasing usage of to-infinitives after 'help'. I'm a bit tied up now, so I'll leave it until tomorrow to follow (and hopefully grok! :) your links. Jul 20, 2021 at 17:41

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