Modal auxiliary verbs

Function

Modal auxiliary verbs express relations (such as permission, obligation, ability, etc.) between the doers of actions and the actions themselves. The singular forms of German modal auxiliary verbs use special stems.

Infinitive	Singular stem	Meaning
dürfen können mögen müssen sollen wollen	darf kann mag muss soll will	to be permitted to, may to be able to, can to like to to have to, must to be supposed to, should to want to

Forms

The present tense forms of the modal auxiliary verbs use a special stem in the singular and the infinitive stem in the plural. The ich- and er / sie / es-forms take no personal ending.

Personal pronouns
Singular	Plural	Singular & Plural
ich kann	wir können
du kannst	ihr könnt	Sie können
er kann sie kann es kann	sie können

Word order

The main verb of action appears at the end of the sentence or clause introduced by the modal auxiliary verb.
In a dependent clause, the modal verb appears at the end.
In compound past constructions, the modal appears at the end as an infinitive, forming a double infinitive construction.
If the compound past construction appears in a dependent clause, the helping verb haben precedes the double infinitive.

1) Ich kann sie nicht verstehen.
2) Es ist schade, daß ich sie nicht verstehen kann.
3) Ich habe sie nicht verstehen können.
4) Es ist schade, daß ich sie nicht habe verstehen können.

1) I can't understand her.
2) It's a shame that I can't understand her.
3) I haven't been able to understand her.
4) It's a shame that I haven't been able to understand her.