Spanish HELP!!!! preterite & imperfect?????????!!!!!!!!!!?

Ok, so I am in my 3rd year of spanish, i am not bad, but this year we have been learning about preterite and imperfect. Not a lot of people in my cl get it and we have a test on it tomorrow!!! Its not just on that, but its part of the test, so can someone give me a good way to remember when to use preterite and when to use imperfect!!!! Gracias