The verb devoir usually means must / to have to (see Conjugate devoir in Le Présent (present tense)).
Devoir in Imparfait = I was supposed to [do]
Actually, in most cases, the obligation was not met.
To conjugate devoir in Imparfait, see Conjugate regular verbs in L'Imparfait (imperfect tense)
Devoir in Passé composé = I had to [do] / I must have [done]
- a past obligation that was met.
- the likeliness of an action having taken place, a hypothesis on a past situation.