It depends which past tense are you referring to. Past Simple is used for actions that are finished. - Shakespeare wrote many plays. (he is dead, so he can't write any more) Past Continuous is used to say that somebody was in the middle of an action at a certain time. - At 10AM I was playing tennis. Past Perfect is used in situations when an event happened before another event took place. - When Sarah arrived home, Tom had already gone. Past Perfect Continuous explains that something had been happening for a period of time before someghing else happened. - We had been running for about an hour when we met Dave. I walked home from work yesterday. (Past Simple) I was walking home when I met Tom. (Past Continuous) When I entered the house, I saw they hadn't cleaned it for months. (Past Perfect) I had been waiting for the bus for an hour when it came at last. (Past Perfect Continuous)

That is pretty large question. Do you have something in particular that you are referring to?