When expressing debt, the verb 'devoir' does not change its meaning for English speakers from tense to tense. So for instance, "Il me doit vingt dollars," "il me devait vingt dollars," etc., translate as "He owes me twenty dollars," "he owed me/used to owe me twenty dollars," etc.

    But when expressing obligation or probability, from the point of view of English the verb 'devoir' can in fact change meaning in different tenses and moods.

    So the present tense tends to means 'must,' 'have to,' 'got to' ("je dois faire des course," "I have to go shopping");

the imperfect 'was supposed to' ("tu devais faire la lessive," "you were supposed to do the wash");

the passé composé 'had to' ("elle a dû téléphoner à son petit ami," "she had to telephone her boy friend");

the future 'will have to' ("nous devrons acheter des billets d'avion," "we will have to buy airline tickets");

the conditional 'should' ("vous devriez vous reposer un peu," "you should get a little rest");

the past conditional 'should have' ("Ils auraient dû rendre les livres à la bibliothèque," "they should have returned the books to the library).

    The adverb 'quelquefois' can be viewed as an expression occupying the middle of a temporal range: 'toujours,' 'souvent,' 'quelquefois,' 'rarement,' 'jamais.'