In the past tenses we already had a short glance at modal verbs. The definition of modal verbs is that modal verbs do not express an action but clarify the semantic value of the main verb (which describes the action of the subject).

Ich muss zahlen. = I have to pay. / I must pay. Ich will zahlen. = I want to pay. Ich kann zahlen. = I can pay.

As you can see in the examples the meaning between the three sentences is quite different even though the main verb is in all three the same.

German modal verbs müssen = to must / to have to sollen = to shall können = to can   dürfen = to may   mögen = to like   wollen = to want to