devoir in l'imparfait can mean "suppose to" and "had to"?

Question

Is my thinking right here? If devoir is used in the imparfait in the main clause it means suppose to, and if it's used in the imparfait in the subordinate clause it means had to, all be it with less certainty than using devoir in the passé composé?  In the text taken from one of your "fill in the gaps" on chosing the imperfect or the compound past :
 "Le père était âgé et sortait rarement de son lit, alors sa fille devait s'occuper du jardin et des animaux."
 the translation is given that she had to take care of the garden which means in english anyway, that she carried out the obligation. In french does the repeated action overule the subtlety of the fulfilled obligation?
I hope that's clear!

Céline Pickard · Accepted Answer

Bonjour Jeramy,
Whether "devoir" is part of a main clause or a subordinate clause, it can mean either "suppose to" or "have to". It all depends on the context and whether the speaker implies the action was actually carried out. Take a look at these 2 examples below (from the lesson):
Ils devaient venir ce soir, mais ils ont annulé à la dernière minute. = They were supposed to come tonight, but they cancelled at the last minute.
-> L'Imparfait here expresses an unfulfilled expectation.
Il a dû le laisser là pour toi. = He must have left it there for you.
-> Le Passé Composé implies a past deduction/supposition.
In "Le père était âgé et sortait rarement de son lit, alors sa fille devait s'occuper du jardin et des animaux.", the narrative context (an aged father who rarely comes out of bed) implies a repeated duty (i.e. the looking after the garden).
I hops this is helpful.
Bonne journée !