As well as in the conjugation for the present tense the conjugation of the pretérito imperfecto is different depending on the ending of the infinitive verb (-ar, -er, -ir). First we need to find the stem of the verb, which is for regulars easy: take the infinitive ending of the verb away. Then the new endings are put in place.

The good news about the pretérito imperfecto is that it is in most cases regular. There are only a few irregular verbs that change the stem in the imperfect tense.