There are two sentences here: “I am supposed to clean my room.” “I should clean my room.” I think the two sentences have the same meaning. That means “be supposed to” and “should” have the same grammatical function. But I am not so positive about that, I still doubt there might be some differences between them. I’d like to know whether they are any differences and how to use them.
My comments are:
Personally, I think the two are not interchangeable, i.e. not the same. The meanings of the two may vary depending on, among other things, the related contexts and the tone of the speaker. "Should" has a stronger tone and refers to something you are obligated to do. As for "supposed to", it mostly refers to something which others think you are obligated to do so but you feel otherwise.
- "I should clean the room"- My mother told me to clean the room, but I did not do it. I know it is my duty to do it. I feel sorry for not doing so.
- "I am supposed to clean the room"- My mother told me to clean the room, but I did not do it. It is only according to my mother I should do it, but I don't think it is my duty to do it.
When you run a stop sign and got stopped by the police, which of the followings you think the police would say to you?
- "John, you should stop at the stop sign." or
- "You are supposed to stop at the stop sign."
My choice would be Sentence 1. A driver is required by law to stop at the stop sing so he should (!!!) stop not only "supposed to" stop. Besides the former sentence makes the speaker sound more authoritative than the latter.