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 go" (not *"I will to go");
- after had better: "You had better ask him";
- with the verb help: "She helped him finish it"; (the to-infinitive is also possible here)
- with why: "Why do it later?".
- with let, make, and sometimes have (see have someone do something)
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.