Infinitive is a grammatical term used to refer to certain verb forms that exist in many languages. As with many linguistic concepts, there is not a single definition applicable to all languages. In traditional descriptions of English, the infinitive is the basic dictionary form of a verb when used non-finitely, with or without the particle to. Thus to go is an infinitive, as is go in a sentence like I must go there (but not in I go there). The latter is called the bare infinitive, the former the full infinitive or to-infinitive. In many other languages the infinitive is a single word, often with a characteristic inflective ending, such as manger ("(to) eat") in French, portare ("(to) carry") in Latin, lieben ("(to) love") in German, etc. Some languages do not have any forms identifiable as infinitives. Many Native American languages and some languages in Africa and Australia do not have direct equivalents to infinitives or verbal nouns. In their place they use finite verb forms in ordinary clauses or various special constructions. Forms identified as infinitives are generally non-finite verbs in most uses. They may function as other lexical categories, such as nouns, within the clauses that contain them, for example by serving as the subject, object or complement of another verb or preposition. As non-finite verbs, they are generally used without a stated subject, and as a rule they are not inflected to agree with any subject; nor do they normally inflect for other categories such as tense, aspect, mood or voice (although such inflection sometimes occurs to a certain degree, for example Latin has distinct active and passive infinitives). Other non-finite verb forms which often share many of the above properties (but are not classed as infinitives) include participles, gerunds and gerundives.

English language has three non-finite verbal forms, but by long-standing convention, the term "infinitive" is applied to only one of these. (The other two are the past- and present-participle forms, where the present-participle form is also the gerund form.) In English, a verb's infinitive is its unmarked form, such as be, do, have, or sit, often introduced by the particle to. When this particle is absent, the infinitive is said to be a bare infinitive; when it is present, it is generally considered to be a part of the infinitive, then known as the full infinitive (or to-infinitive), and there is a controversy about whether it should be separated from the main word of the infinitive (see Split infinitive). Nonetheless, modern theories typically do not consider the to-infinitive to be a distinct constituent, instead taking the particle to for operating on an entire verb phrase; so, to buy a car is parsed as to [buy [a car]], not as [to buy] [a car]. The bare infinitive and the full infinitive are mostly in complementary distribution. They are not generally interchangeable, but the distinction does not generally affect the meaning of a sentence; rather, certain contexts call almost exclusively for the bare infinitive, and all other contexts call for the full infinitive. Huddleston and Pullum's Cambridge Grammar of the English Language (CGEL), published in 2002, does not use the notion of the infinitive, arguing that English uses the same form of the verb, the plain form, in infinitival clauses that it uses in imperative and present-subjunctive clauses.

The full infinitive (or to-infinitive) is used in a great many different contexts: Outside of dictionary headwords, it is the most commonly used citation form of the English verb: "How do we conjugate the verb to go?" It can be used like a noun phrase, expressing its action or state in an abstract, general way. So, "To err is human"; "To know me is to love me". (However, a gerund is often preferred for this — "Being is doing" would be more natural than the abstract and philosophical sounding "To be is to do."[1]) It can be used like an adjective or adverb, expressing purpose or intent. So, "The letter says I'm to wait outside", or "He is the man to talk to", or "[In order] to meditate, one must free one's mind." In either of the above uses, it can often be given a subject using the preposition for: "For him to fail now would be a great disappointment"; "[In order] for you to get there on time, you'll need to leave now." (The former sentence could also be written, "His failing now would be a great disappointment.") It can be used after many intransitive verbs; in this case, it generally has the subject of the main verb as its implicit subject. So, "I agreed to leave", or "He failed to make his case." (This may be considered a special case of the noun-like use above.) With some verbs the infinitive may carry a significantly different meaning from a gerund: compare I stopped to talk to her with I stopped talking to her, or I forgot to buy the bread with I forgot buying the bread. It can be used after the direct objects of many transitive verbs; in this case, it generally has the direct object of the main verb as its implicit subject. So, "I convinced him to leave with me", or "He asked her to make his case on his behalf." However, in some cases, the subject of the main clause is also subject of the infinitival clause, as in "John promises Mary to cook", where the person who will cook is John (the subject of the main sentence), and not Mary (the object). As a special case of the above, it can often be used after an intransitive verb, together with a subject using the preposition for: "I arranged for him to accompany me", or "I waited for summer to arrive." When the verb is implied, some dialects will reduce the to-infinitive to simply to: "Do I have to?"