When do I use the preterite and when do I use the imperfect?

If you imagine a timeline (would draw out a line on the whiteboard here), the imperfect appears at some extended period over this. It is important to remember that the imperfect encompasses the ideas: something that you used to do, something that you were doing. Eg. I was playing football - Jugaba al fútbol. I used to play football - Jugaba al fútbol. On the same timeline, the preterite would come at one specific point. The imperfect is a continuous act, the preterite is a definitive act. Eg. I was playing football when my mum arrived - Jugaba (imperfect) al fútbol cuando llegó (preterite) mi madre. Eg. My mum was arriving when I said hi - Mi madre llegaba (imperfect) cuando yo dije (preterite) hola.