What is the difference between the pretérito and the imperfecto?

When you think of an action in the past, you must ask yourself: did it happen once, in a fixed time frame (where the beginning and end are specified) or did it happen multiple times (where no beginning or end is specified). The principle difference between these two past tenses is that one happened once (pretérito) and the other happened frequently (imperfecto). For example, if I used to play with my brother when I was younger (De niño, jugaba con mi hermano) you can see two things that tell you which tense you must use in Spanish. 1) used to, which implied repeated actions. 2) with my brother, as this doesn’t specify a beginning or an end, but simply adds details to the action.An example of the pretérito is: Yesterday, I had my hair cut (Ayer, se me ha cortado el pelo). Here, you should pay attention to two things. 1) Ayer, indicating an action has recently taken place. 2) the action of getting one’s hair cut is itself inherently singular- unless your hair grows back overnight!