The dictionary form of a verb, usually preceded by “to,” as in “to walk,” is called the infinitive.
The form with “to” is known as the to-infinitive: “to walk,” “to read,” or “to understand.” It can act in several ways within a sentence, including as a subject, complement, or purpose phrase. For example, “To travel is exciting” uses an infinitive phrase as the subject, while “She went to learn” expresses purpose.
English also has a bare infinitive, the verb form without “to.” Modal verbs require it, as in “can walk” and “should read,” and it also follows verbs such as “make” and “let” in constructions like “Let him go.” This is why “to” is common but not required in every infinitive.
The word comes from Latin infinitivus, meaning “unlimited” or “unfinished.” An infinitive is not the same as a gerund: “To swim is healthy” uses an infinitive, while “Swimming is healthy” uses an -ing form functioning as a noun. The phrase “split infinitive” refers to placing a word between “to” and the verb, as in “to boldly go.”