Couldn't, Wouldn't, You'd, You'll, etc...

[the_p0et]

I'm doing language exchange with a couple University girls. One girl understands about half of what I say while the other girl still has a forgotten elementary level of English. It's hard to teach them English together with two different levels. They want to write the TOEFL or TOEIC exam eventually.

I was trying to explain when/why to use: Won't (instead of "will not") and the same for Can't, Wouldn't, Couldn't, You'll, You'd, Shouldn't and "You'd better...". These were in one of their own books and they didn't know what they meant and why to use: "Can" you pass the butter? instead of "Could" you pass the butter? or "Would" you pass the butter? or "Will" you pass the butter? etc...

How can I explain these words to them in the simplest English possible? (i wish I knew more Chinese!) haha

Thanks!