How do you form the imperfect tense in French?

To form the imperfect tense, you take the -nous form of the present tense and remove the -ons ending. You then add the following endings depending on the subject. 

je = ais

tu = ais 

il/elle/on = ait 

ils/elles = aient 

nous = ions

vous = iez

For example, I was asleep when she came home = Je dormais quand elle a rentré