What is the difference between the simple past (preterite) and the imperfect tense?

Spanish, unlike English, has two different past tenses, the simple past, or preterite, and the imperfect. The two have different meanings and are conjugated differently.

A verb in the simple past means that an action happened at a fixed period of time. The action is completed and it all happened in the past. For example, "Yo fui al cine" (I went to the cinema) or "Él habló con Pablo ayer" (He spoke with Pablo yesterday). Generally, if you see expressions of time, such as "ayer", "la semana pasada", etc., the sentence will use a simple past.

A verb in the imperfect is something that happens in the past, not at any particular point in time, or habitually over time. Many times, this can be translated into English as "used to", but not always. This tense is used to set the scene and describe how something was. For example, "Cuando vivía, era profesor" (When he lived, he was a teacher) or "Mi madre siempre me decía que tenía que comer las verduras" (My mother always told me that I had to eat my vegetables).

Often times you will see the two past tenses in the same sentence. The imperfect will set the scene and the simple past will tell you what happens in that moment. This will happen a lot in stories. For example, "Mientras hablábamos sobre algo importante, mi hermano nos interrumpió" (While we were talking about something important, my brother interrupted us). The first verb is setting the scene generally, and the second one describes an action that happened right then and there and was completed. This sort of sentence is good for helping you remember the difference.