Ashley, The choice of which tense you would use is based on the concept of "relative time." In other words, it depends on whether you are trying to express a relationship between an event in the past and what exists in the present, or between two distinct events which occurred at different points in the past. It also would be based upon how long each of these events lasted, and whether they were single or repeated events. This is WAY to complicated to explain here, but if you want, then please send me a private message, and we can discuss this on Skype or on the phone. I think that would be the easiest way for me to make it (more) understandable.