When do you use the imperfect tense in Spanish?

In Spanish, the imperfect tense is used to describe the past. It is used in 3 ways. 1 - to describe something that used to happen (I used to go to dance classes) 2 - to give background information about something (The city was old and pretty)3 - to describe something that was happening, when something else took place (I was cooking when the phone rang)eg. 1 - Iba a clasa de baile. 2 - La ciudad era antigua y bonita. 3. - Cocinaba cuando sonó el teléfono. In the last example, you can see that both the imperfect and the preterite tenses are used. It can be useful to think of this use of the imperfect tense like the spikes on a heart rate monitor. The long line at the bottom is the imperfect tense (an action that was taking place in the past). The spikes are single actions that interrupt the initial action. For example, 'I was cooking' would be the long line at the bottom (the imperfect tense), and 'the phone rang' would be a spike (the preterite tense), because it is a short, finished action which interrupted my cooking!