Hello, Theomax.
When you ask for help with an exercise we want you to give your 'try'.
Here are the possibilities, written out in full:
1. I were watching TV when Paul and Simon arrived.
2. I am watching TV when Paul and Simon arrived.
3. I was watching TV when Paul and Simon arrived.
4. I is watching TV when Paul and Simon arrived.
Which one looks best to you? Which ones do you think are probably wrong? You don't have to be correct; just do your best.
Once people know your thinking, they will know what you understand and what still needs to be explained --- if anything does.
