The grammatical term for a verb’s base form, usually preceded by “to,” is infinitive.
In English, the to-infinitive consists of to plus the plain form of a verb: to walk, to understand, or to be. Infinitives are non-finite verb forms, meaning they do not normally show tense, person, or number. They can act in several roles: “To travel is exciting” uses an infinitive as a subject, while “She hopes to travel” uses one after another verb.
The word “usually” matters because English also has the bare infinitive, which appears without to. Modal verbs require it in sentences such as “They can swim” and “You must leave.” It also follows verbs such as let and make: “Let him go” and “She made them laugh.”
Infinitives are often confused with gerunds and participles, which commonly use an -ing form. In “I like to read,” to read is an infinitive; in “I enjoy reading,” reading is a gerund functioning as a noun. The term comes from Latin infinitivus, meaning “unlimited” or “uninflected.”