Modal Verbs

You mustn't forget certain things, such as: 

- no "s" in the 3rd person singular

- in the interrogative form we need "Inversion" (May I .....?)

- you do not need an auxiliar verb to write the interrogative or the negative forms

- Modal + verb in the infinitive (to)

This table was taken from a book called "Exercícios de Inglês 11º" by Porto Editora.I believe it is an excellent table to synthesize the Modal Verbs.