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