Irregular imperfect tense forms
The imperfect tense of practically all Spanish verbs conforms to the 'regular' -ar and -er/-ir patterns: even verbs which are irregular in other tenses generally fall into these patterns. There are just three Spanish verbs that have irregular imperfect tense forms. These verbs, and their imperfect tense forms, are:
In the verb ver, we can see the e as being both part of the infinitive ending -er and the stem. Then we can view the imperfect forms as keeping the vowel of the stem and just removing the final -r (and other than that, they are perfectly regular).
Practise irregular imperfect tense forms
In the exercise below, you can practise these irregular forms. For the sake of practising these forms, translate used to be using the imperfect of ser. (In reality, of course, used to be would in some cases be translated with the imperfect of estar.)