Theorie:
Study the information about modal verbs in general.
Modal verbs are a part of the larger category called auxiliary verbs, which are verbs that cannot be used on their own. They need to be accompanied by another (main) verb.
Can, could, may, might, must, shall, should, will, would - modal verbs
Modal verbs are used to express ability, obligation, permission, assumptions, probability, possibility, requests, offers and advice. Each modal verb can have more than one meaning, which depends on the context of the sentence (or question).
You can go now. (= permission)
I can play the guitar. (= ability)
Structure with Modal Verbs.
A modal verb is followed by another verb in the base form without any changes and endings.
Subject + Modal Verb + Verb (base form of the infinitive)
I can run fast.
She can swim very well.
Modal Verbs in Negative Sentences.
Subject + Modal Verb + not + Verb (base form of the infinitive)
You must not walk on the grass.
He cannot speak Spanish.
We should not read this document.
The negative of can is cannot ('not' is joined to 'can') and the contraction is can't.
Modal Verbs in Questions.
Modal Verb + Subject + Verb (base form of the infinitive)
May I help you?
Can I have another piece of cake please?
Quellen:
http://www.grammar.cl/english/modal-verbs.htm