be supposed to为“半助动词”，意义与情态动词should或ought to相同。be supposed to不宜拆分，常被视为习语，整体地当作助动词，后接动词原形一起构成句子的谓语（也有人称其为“复合谓语”）。夸克《英语语法大全》（P.193）将此类结构视为“半助动词”，摘录如下：

半助动词：be going to等

3.14 半助动词包括一批动词习语，它们具有情态或体的意义，并由动词HAVE或BE引导；例如：

be able to; be bound to; be likely to; be supposed to; be about to; be due to; be meant to; be willing to; be apt to; be going to; be obliged to; have to

半助动词范畴的边界不明确：可以扩大到把be unable to, be unwilling to等否定习语也包括进去。……

你自己仔细体会词典（韦氏高阶）对be sopposed to的不同释义和例句，想想每个例句是否可以用should替换，替换后意义是否相同。

be supposed

1 

a : to be expected to do something

• They are supposed to arrive tomorrow.

• She was supposed to be here an hour ago.

• The movie was supposed to earn a lot of money at the box office, but it didn't.

b : to be intended or expected to be (something)

• The party was supposed to be a surprise.

c — used to show that you are angry or offended by something— followed by to + verb

• Was that supposed to be funny? I thought it was quite rude.

• “Well, you've done it again.” “What's that supposed to mean?” [=what do you mean by that comment?]

2 

a — used to say what someone should do — followed by to + verb

• You are supposed to listen to your parents.

• I'm supposed to clean my room before I go outside.

• What are we supposed to do [=what should/can we do] in a situation like this?

• Do what you're supposed to.

b — used to say what someone is allowed to do — followed by to + verb

• We were not supposed to leave the room.

• Are you supposed to be here after the building has closed?

3 

— used to indicate what people say about someone or something— followed by to + verb

• She is supposed to be the best doctor in town. [=people say that she's the best doctor in town]

• That breed of dog is supposed to be good with kids.

• The word is supposed to be derived from Latin.