majid,

I agree with Steve's comments. Some rules to follow when using "thinking of" and "thinking about" are:

1. If you are imagining something, use "think of"
2. If you are considering something, use "think about"

For 8: I had never thought of becoming an actor.---> To me, this means that you are not an actor now, and you had never had thoughts regarding taking this career path. If you were to say "I had never thought about becoming actor," I jump to the conclusion that right now you ARE an actor, but you had previously not considered this as a career choice.

Another example: If I say "I need to think of the solution to this problem," I mean that I need to find an answer. But, if I say "I need to think about the solution to this problem," I mean that a solution has already been proposed and I need to consider whether or not it will work/is practical/etc.

It seems to me that one can use "about" to convey the idea of something that is concrete. One thinks about the past or thinks about issues.

I hope that this helped!