The Imperfect tense expresses potential action or a non-factual action from the viewpoint of the past tense. In English, we would use might and would for translations.

For Example: Caesar went to Gaul in order that he might attack the enemies.

Voco, Vocare, Vocavi, Vocatus: You take the present infinitive (Vocare) and add the appropriate ending. So Vocarem would mean "I would call" and Vocareris would mean "You might be called".