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)