The past (also known as preterite) is a grammatical tense to make statements about the past. In German it isn't often used in speech. It's more common in written language, such as in newspaper articles and books.

Modal verbs are an exception. They're frequently used in the simple past tense. Just like in the present tense, the modal verbs often indicate something about the relationship of the subject to the action expressed by a second verb in the sentence - the main verb.

Examples:
Wir wollten nach München fahren.
Ich musste auf die Toilette gehen.
Ich konnte nicht ohne Helen abfahren.
 

How to form it
Forming the simple past is the same for all the modal verbs:

Note that "können", "müssen" and "dürfen" lose their umlauts.

Word order in sentences
Just like in the present tense, modal verbs in the past tense are conjugated and placed in a sentence according to the regular rules for conjugated verbs. For simple declarative sentences and "W" questions, that means they come second. The main verb - in the infinitive form - is placed at the end.