I use 'supposed to' to differentiate that quite often the action isn't done. eg. 'You are supposed to wear a helmet on a bicycle' meaning - in reality a lot of people don't.

i think i get it,thank you for teache me!:)

Should means that it really is the best choice. "Supposed to" means that it is following the rules. ("You should have given him the book" means that it would have been your best course of action. "You were supposed to give him the book" means that the agreement, expectation or rule was that you give him the book. But the second sentence does not mean that you erred by not giving him the book--could be the speaker thinks you are better off keeping it for yourself!)

I agree that they can be used interchangeably in most cases, but I would say that there is a slight difference between the two phrases in some case. For instance, sometimes "supposed to" is used to make the action stated sound like something that you had to do while "should" can make it sound like you had more options in what action you took, and decided against the option that would have been a better choice. For example: -You are supposed to take out the trash. It is your responsibility. (In this case, you are expected to take out the trash. It is not a suggestion). vs. -You should take out the trash. It is your responsibility. Besides, if you don't take out the trash, the house will begin to smell. (In this case, it is more of a suggestion. It is the right thing to do since it is your responsibility, so it would be best if you did it. You don't necessarily have to, but if you don't, the house will smell.) It's a tad bit confusing, but the more you practice speaking with people the easier it is to pick out the few cases were there is actually a bit of a difference between the two. If you want to keep it simple though, you can continue to use them interchangeably and, in most cases, you'll be okay.

As Miri said, they can be used interchangeably. Personally I tend to use them for tenses, "should" for future, "supposed to" for past. There are some who may use "supposed to" in a more accusative formal way "you were supposed to buy me flowers!". Supposed to can also show that someone else is relying on you to do something.