How do I know when to use the Imperfect Tense in Spanish?

In general, the imperfecto is used to talk about past actions, conditions, or what you would consider background information. It can also be used to describe events that occured repeatedly or habitually or that were continuous. If one ongoing action is interrupted by another, shorter one, you use the imperfect to describe the longer action. For example, the sentence "I was washing my hands when the telephone rang." would be "Lavaba mis manos cuando sonó el teléfono." in Spanish. As I mentioned, it is also used to talk about "background information", so things like dates, a person's age, feelings or certain characteristics in the past are best described using the imperfect tense. Most times, if you would use the expressions "used to" or "would" in English, you use the imperfecto in Spanish. For example, if you were to say "I used to play the piano when I was younger.", you can translate this as "Tocaba el piano cuando era pequeño."