When is the imperfect tense used and how do you form it?

The imperfect tense is used to express the idea that something is happened continuously in the past. It translates in English as "I was X-ing".
To form the imperfect tense, you take the present tense nous form of a verb (e.g. avoir = nous avons), and take aways the -ons ending, giving you the stem for the imperfect tense.then you add these endings:-ais-ais-ait-ions-iez-aient
e.g avoir = nous avons, so the stem of avoir is -AV
Elle avAIT = she was having