1.be supposed to do= should do 应该做某事：
you are supposed to go to school on time=you should go to school on time.
你应该按时上学.
2.be supposed to have done sth= should have done.应该做成了某事（实际上没做成）：
You are supposed to have done your homework= you should have done your homework.
你本应该能完成作业的.