Could, should, would
I could go.
I couldn’t go.
I should go.
I shouldn’t go.
I would go.
I wouldn’t go.
Could I go?
Should I go?
Would I go?
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.
It could help me.
You should give me a reasonable order.
I would be happy if I could walk very slowly toward a fountain.
You can see more examples in the book The Little Prince.