modal verbs
Verbs such as can, could, may, might, must, shall, should, will, and would.
A modal verb sits with a base verb and marks possibility, permission, obligation, or tense coloring: can file, must wait, should sign. Modals do not take -s in the third person: she can, not she cans. They take a bare infinitive, not to: must go, not must to go. See also may / might and can / may.
Examples
UseShe must go.
AvoidShe must to go.