The Queen won't come to dinner this Saturday. Simple future tense, right? Simple fact. You have not invited her. So it is a simple statement of fact. The Queen would not come to dinner this Saturday even if you invited her. Pure hypothesis. You have not invited her, and even if you did, she would not come. The whole thing is unlikely to happen. If I were you, I would use a grammar book. Again pure hypothesis. I could never be you. If I were you, I wouldn't smoke. If I had a million, I would give it all to you. It is highly unlikely that I would have a million. So I am just saying it to make you happy. If I have a million, I shall repay my loan right now. It's a genuine intention. I don't want to have the burden of a loan any more.

you use won't when you are sure that you will do a certain thing.. and you use would't when you wish to do that thing but are not sure about that...

Tip: "What is the difference between ___ and ___?" Use this structure when asking about differences.