Haya, first off, don't worry about not being accepted. The whole point of taking the CELTA is to learn more about how to teach English. From what I can tell based on your answers you're right where you should be. Remember, the objective of the pre-task is to give you a chance to show the committee what you know already, not what you should know already.
#1 You could explain these by giving them a context, a situation where you personally would use them:
If I had the money, I would buy a new car.
If I have the money, I will buy a new car.
#2 A person on desert island is isolated if s/he is lonely; however, if s/he isn't lonely, say, chooses to be away from everything and everyone, like a famous person who goes on vacation, then the island would be considered secluded, away from everything and everyone. You could explain this to the students and then have them brainstorm situations where people and/or things are isolated and secluded.
The house was very isolated.
The house was very secluded