Could, should, would
When we want to say that something is possible, we use could. For example I could do it. I could be the best. She could help me.
When we want to say that it is good to do something, we use should. For example You should do it. I should study more. He should help us.
When we speak about a situation which can happen under some condition, we use would. For example I would pay if I had money. I would tell you if I knew it.
You can see more examples in the book The Little Prince.