Bare infinitive

A bare infinitive (/ˈbeər ɪnˈfɪnətɪv/) or zero infinitive is an English infinitive; its form simply consists of the base form of a verb; unlike a to-infinitive it lacks the particle “to.”

Form

The bare infinitive is used:

• as the main verb after auxiliary do do, e.g. "eat" in “Do you eat pizza?"
• as the main verb after a full modal verb: “I will eat" (not *“I will to eat");
• With some semi-modal verbs:
  • after had better: You had better ask him;
  • with need and dare, e.g. I need eat, though this is archaic and the to-infinitive is used instead.
• with why: “Why do it later?" (not *Why to do it later?)
• with causative verbs let and make; I made him eat.
  • sometimes with have as part of the structure have someone do something
  • with the verb help: “She helped him finish it"; (the to-infinitive is also possible here).
  • with bid;
  • with have never known, in BrE.
• with verbs of perception in the active voice, e.g. I saw him enter the room.

Pedagogy

In English, the bare infinitive is the exception rather than the default; the to-infinitive is the default. Consequently, the exceptions listed above have to be acquired (or learnt) as exceptions. EFL learners will often make errors by substituting the to-infinitive, particularly after let and make.

Unfortunately, in pedagogical grammar, verbs are sometimes referred to as “bare infinitives" despite their being finite verbs (i.e. possessing a subject), and therefore categorically NOT infinitives. For example, in “I eat pizza,” "eat" has a subject (“I") and is finite and not an infinitive.