Modal verbs are auxiliary verbs (also called helping verbs).

After an auxiliary modal verb, the root form of the main verb is generally used, e.g., I can play the piano. 

An infinitive verb should not appear after a modal verb, e.g., I can to play the piano.

I can to play the piano. INCORRECT

I will to play the piano. INCORRECT

I could to play the piano. INCORRECT

I shall to play the piano. INCORRECT

I must to play the piano. INCORRECT

I would to play the piano. INCORRECT

I may to play the piano.               INCORRECT

I might to play the piano. INCORRECT

I should to play the piano. INCORRECT