Learn the past stems
Common past forms include konnte, musste, durfte, wollte and sollte. The first and third singular forms match. A full pattern is ich konnte, du konntest, er konnte, wir konnten, ihr konntet, sie konnten.
Ihr konntet früher anfangen.
You were able to start earlier.
The action remains an infinitive
The modal carries the tense and person ending. The action verb remains an infinitive, normally at the end of the main clause. A separable infinitive stays in one piece.
Wir mussten die Tür zumachen.
We had to close the door.
No obligation is not a prohibition
Ich musste nicht warten says waiting was unnecessary. Ich durfte nicht warten says waiting was not permitted. Keep the meaning of the modal when adding negation.
Du musstest nicht bezahlen.
You did not have to pay.
Konnte is not könnte
The umlaut matters: konnte is a past form, while könnte commonly expresses a hypothetical possibility or a polite suggestion. A present infinitive with an umlaut does not imply an umlaut in the past stem.
Damals konnte ich gut schwimmen.
Back then I could swim well.
With weil, close the clause with the modal
In the simple subordinate pattern, the action infinitive precedes the finite modal: weil wir warten mussten. This is separate from more advanced perfect modal constructions.
Wir blieben dort, weil wir auf Anna warten mussten.
We stayed there because we had to wait for Anna.
Practise this pattern
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References
Original teaching prose and examples, informed by these grammar references. A2 placement is an editorial curriculum choice, not a CEFR certification.