The verb devoir (to have to) is used in Conditionnel Présent to express should.
It uses the same irregular stem as in Futur Simple: devr-
Have a look at these sentences:
Here's how to form the Conditionnel Présent of devoir:
devr- + endings of Imparfait: -ais, -ais, -ait, -ions, -iez, -aient