1.be supposed to 表示的是一种预期 (expectation) 的情况：
(1)现在式的 be supposed to v. 指的 “应该”, 则是指预定的计画, 或别人预期我们要做到的事
(2)过去式的 be supposed to 意为「大家预期发生的事，但实际上却没发生」，因此相当于 should have + P.P. 或 ought to have + P.P. 「本该发生的事，但实际上却没发生」。
A: The train is supposed to be on time.
B: Yeah, it’s never late.
A: Johnny, you are supposed to finish your homework before you watch TV!
B: Of course, mom. I will finish my homework first before I watch cartoons.
A: You were supposed to call that girl! I even gave you her phone number!
B: I’m sorry. I just didn’t have the courage to call.
A: Class was supposed to begin at 10: 00, but it didn’t begin until 10: 15.
Class should have begun at 10: 00 (but it didn’t begin until 10: 15).
Class ought to have begun at 10: 00 (but it didn’t begin until 10: 15).
2.be supposed to 除了上述的意思外，还有一个常见的意思，就是「一般认为」(be generally believed/considered)
a.Elephants are supposed to be afraid of mice.
b.Elephants are generally believed to be afraid of mice.
c.It is generally believed that elephants are afraid of mice.
(Yes, I know that elephants aren’t really afraid of mice.)
3.should/ought to 指的 “应该”, 是说做这样的事是有好处的 (ought to 的语气比 should 来得强)
a.You should go get new glasses. You can’t see clearly with the ones you have now.
b.You should study hard now. The final is only a few days away.
或者表示义务, 责的任应该, 应当
a.You should help these people. It’s your duty as a doctor.
b.A teacher should also dedicate himself or herself to education.
