konnte, musste, wollte, durfte

The modals können, müssen, dürfen, wollen and mögen have a weak preterite: stem plus -te, with the umlaut removed. So können becomes konnte, müssen becomes musste, dürfen becomes durfte, mögen becomes mochte (note the change from g to ch), and wollen becomes wollte. These forms are the normal past of the modals in spoken German, unlike most other verbs, which usually use the perfect in speech.

The infinitive of the main verb goes to the end of the clause: Ich musste den ganzen Tag im Bett bleiben. A common mistake is keeping the umlaut: Ich müsste arbeiten is Konjunktiv II and means I would have to work, not I had to work. Another error is putting the infinitive in position two instead of at the end.