The imperfect tense (imperfetto) is another form of the past tense. l'imperfetto is a simple tense – meaning it is formed using one verb. (Compare this to a compound tense like the passato prossimo which is made from a combination of two verbs.)
Like the passato prossimo, the imperfetto is used to narrate events in the past. The main difference between the two is the imperfect is used to describe ongoing past actions, while the passato prossimo is used for completed past actions.
The graphic below is a visual representation of the relationship between the imperfect and passato prossimo:
The imperfect is depicted as an ongoing past action taking up an unspecified amount of time and the passato prossimo is a completed "done & over" action.
To learn how to form the imperfect tense in Italian, watch the following video:
The following is a set of exercise flashcards that you can use to study the formation of the imperfetto: