The six German modal verbs are: duerfen, koennen, moegen, muessen, sollen and wollen.

They are a few irregularities with their conjugations but common trends can be spotted;

In the imperfect, they follow -te, test, te, ten, tet, ten, ten.

As they are auxiliary verbs, they will be used with another verb:

Ich moechte eine Tasse Kaffee trinken.