Your understanding of the word "would" is wrong. When talking about future events the difference between "will" is referring to something that is going to happen in the real world, but "would" is talking about what would happen in a hypothetical situation. So for example, I could say
I would go skiing with you if I had the money.
and that means that if I had more money (an imaginary situation) then I would go with you. Let's take another sentence.
I would go skiing with you if I could get a day off of work.
This means that I have given up trying to get a day off, and I don't think it's possible. In a hypothetical where I do get off work, I would go with you, but in the real world I will not. However, if I say
I will go skiing with you if I can get a day off of work.
that means that I don't know if I can get a day off or not. I'm still trying to convince my boss to let me have a day off, and I don't know if he will let me or not. If he does, I will go with you, but if he doesn't, I will not.
But what if I want to say "It will not happen?" Because the meaning of "it will not happen" is that something is that is unlikely or, worse still, not going to happen, shouldn't it be more accurate to say "it would not happen"?
If I say "It will not happen", it does not mean it is unlikely. It means "In the future, this event is going to not happen." "It would not happen" is only accurate if you're talking about a hypothetical.
Now onto your last sentence:
"I think you will not be able to do it" or "I think you would not be able to do it".
It all depends on the context.
Let's say that I want to run a marathon. I sign up, and start training. The big day comes and I'm really excited. You want to discourage me so you say:
I think you will not be able to finish the race.
New scenario: Let's pretend that I've always wanted to run a marathon, but I am afraid of failing. I want to try, but I want to know that I can finish the race before I make an attempt. I come up to you and ask, "Do you think that I could run a marathon?" You want to discourage me so you say:
I think you would not be able to finish the race.