How do I know when to use the perfect, the preterite, and the imperfect?

Perfect: As with English, the perfect is used for more recent events - He comido = I have eaten. You would use this probably for something you've done more recently in the day. It can also describe actions which are still going on - He jugado al tenis for un año = I have played tennis for a year (and still am).Preterite: The preterite describes a one off event anywhere in the past - Fui al cine = I went to the cinema, once.Imperfect: The imperfect describes actions which happen over a period of time - they may be repeated actions or ongoing. Often the English equivalent is "I used to" - Iba al parque con mis abuelos = I used to go to the park with my grandparents. However, this isn't always true - one could say "La semana pasada, llovía" for "Last week, it rained" - the imperfect here shows that the rain, rather than happening on one occasion, was happening over a long period of time.