Why is there no 了 in 以前我住在这儿
This will not answer your question but shed some light to the mystery of the 了 particle.

了 does not mean past tense. It denotes **CHANGE**.


以前我住在这儿。 
This sentence points out no change. It only says that once you were living there.

Your friend calls from the kitchen: 吃饭了！ 
This one points out a change, namely that we are ready to eat the meal. It is not past tense, it's in the present, but a change has occured, so wo use 了.

行了！ 
You can use this sentence to say 'Fine with me!'. There is absolutely no past tense here, it just says that I have made up my mind, so now we have a change from the previous undecided situation to one with a resolution.

别吵了！  


Stop quarreling! Here you see, an imperative really has nothing to do with past tense, yet there is a 了 in the sentence because the speaker orders the ones he/she speaks to, to make a change from quarreling to silence.