When to trade

In the lessons it says that if your bishop or other pieces are "good" and by that they mean if the pieces are not blocked by something, you don't trade. This game that I played shows that sometimes you have to trade in order to not be checkmated.

Trading is good when it forces something bad to happen to your opponent. The following game is played many times and trades the queens.

Another thing is when you have more pieces than your opponent you can trade with him. The following game shows why it is good

It was good to trade in this game because white had more pieces.