1.be supposed to 表示的是一种预期 的状况：
目前式的 be supposed to v. 指的 “应该”, 则是指预定的计画, 或其他人预期大家要做到的事
过去式的 be supposed to 意为「大伙预期发生的事，但事实上却没发生」，因此等于 should have + P.P. 或 ought to have + P.P. 「本该发生的事，但事实上却没发生」。
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 .
Class ought to have begun at 10: 00 .
2.be supposed to 除去上述的意思外，还有一个容易见到的意思，就是「一般觉得」
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.
3.should/ought to 指的 “应该”, 是说做如此的事是有好处的
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.
