The infinitive in English is the form that we use to refer to a verb generally. It is the form that we use to name a verb or in making a list of verbs. If we ask a question about the meaning of a verb, for example, we usually refer to it by its infinitive form, as in “Do you know what obviate means?” or “What is the meaning of ionize?”
The infinitive is a form of a verb that does not show time; that is, it does not show past or non-past time by its form. If we say “He wanted to see that movie,” the verb form “wanted” shows that it refers to past time, but “to see,” which is an infinitive, does not reflect past time. If we say “He wants to see that movie,” we know that “wants” refers to present time, but the infinitive “to see” is exactly the same as in the sentence with “wanted.” Because the infinitive is a simple form of the verb and does not show time, it is used in English to represent all verb forms related to it, which is the reason that it is used as the representative form of a verb in dictionaries.
Within sentences in English, an infinitive is generally preceded by the word “to”; for example, “to go,” “to watch,” “to ride,” “to think,” “to sleep,” “to write.” This helps to distinguish it as an infinitive because the infinitive form is identical to many other grammatical forms in English. An infinitive that is not preceded by the word “to” is often called, simply, an “infinitive without ‘to'” but it is also called a “bare infinitive.” It is the bare infinitive that is used to list verbs in dictionaries and it is a required form after most modal auxiliaries (“You must sign it”; “They will decide soon”) and it is required after certain verbs (“He made them wait,” “I let them sleep”).