The first sentence is showing a process. And the second one just tells about the fact in past.

Maybe I can help with a couple of examples: 1. Yesterday, we were playing football when a dog came over and bit me! 2. I like playing football. We played football yesterday.

How about... "Yesterday we were playing football when it started to rain." (Shows the action first. Then something stopped / interrupted it. It relates to another action that happened at the same time.) "Yesterday we played football. (Then) we went to the bar and drank some beers." (Separate actions that aren't really related to each other.)

"Yesterday, we were playing football" VS. "We played football yesterday"? 1- In the first sentence " there was an action in progress", like: Yesterday i was working on my new painting when he arrived. In this sentence sth has happened and finished while sth else was in progress. 2- in the second sentence; "we played football yesterday.", the speaker tells us about a fact nothing more. ;)

If you tell someone,"Yesterday, we were playing football" that person will probably expect you to elaborate. They might say,"and?" But if you say,"We played football yesterday." The person may say, "cool." :P