Hi Plesehelp,
Maybe this scenario can shed some light:
In the office coffee room:...
A: (pouring a cup of coffee while talking to B) I've heard that someone in the company has started a hiking club.
B: Oh! you didn't know! It's Paul, the manager in shipping. I joined his club last month. Sam and Mary in accounting.
A: If I had known that earlier, I would have joined you guys. I like the outdoor.
B: Paul has mentioned that he would like to keep the memebership under 12. You may want to contact him now before someone takes the spot.
This scenario pretty much covers the questions you have postedso far.
Present perfect - used when the context has no past time marker. (yesterday, a years ago, last week, this moring etc). i.e. I have seen the movie twice already.
Simple past - used to convery a simple thought / event already happened.
Mom! Susan ate my pizza.
It rained here yesterday.
I had nothing to do, so I went to a movie.