Modal verbs are special verbs which express different kinds of modality: ability, obligation, prohibition, possibility, advice or permission.

     They use the particle 'not' to form the negative: 

     I can't go to the party.

     She shouldn't arrive late.

     They are used as auxiliaries and placed at the beginning in questions: 

     Can I go to the toilet?

     Could you see the films?

     They don't usually have other tenses. Only can (could, in the past, and be able to for other tenses) and have to (which has all the tenses). But should or must don't have other tenses.

     Modal verbs are always followed by an infinitive without to, except have to:

     She can run fast.

     They should be careful with the pet.

     We have to pay for the trip.