A full infinitive is the base form of the verb preceded by “to” (e.g., “to drink”). A bare infinitive, on the other hand, is simply the base form of the verb without “to” (e.g., “drink”).
Infinitives can be used as subjects, direct objects, adverbs, and adjectives. Full infinitives are more common than bare infinitives, and several verbs can only be followed by full infinitives (e.g., “I promise to visit” not “I promise visit”).