Modal verbs

modal verb: A modal verb never changes. No -s in the third person, no to after it, no do to make a question or a negative. He can swim, she must go, they should wait — the same word for everybody, and the verb after it is always bare.

bare infinitive: The bare infinitive is the verb with nothing attached: go, not to go. Every modal is followed by it. This is the rule behind the most common mistake in the whole family: I must to go has one word too many.

have to: have to is not a modal: it is an ordinary verb, so it does change and it does need do. That is exactly why it is useful — it lends the modals the forms they do not have: I had to go, do you have to go, I will have to go.