may

Tung Quoc wrote:

1.He may have to pay. 2.He may must pay. Both are correct? (may in (1) and (2) expresses degree of certainty, have to and must express necessity) Quoc

1.He may have to pay. (This sentence is correct. It does not indicate that he will have to pay.)

2.He may must pay. (wrong) (Should be He must pay. This sentence indicates that he will have to pay.) 

I have never seen two modals consecutively in any correct sentences.

have to is not a modal; it is a semi-modal.

Again, "Are both correct?"

You need to think about question formation.