To form modal verbs in the past, it is much more common to use the imperfect tense than the perfect tense.
The modal verbs are wollen, sollen, müssen, mögen, dürfen, können.
The imperfect endings for the modal verbs follow this structure:
Umlauts are dropped from the stem of the modal verbs if they had one in the infinitive form, eg:
Here are some examples of the modal verbs in the imperfect tense: