Should

Should是基础，其他三词都以should为标尺对比理解

shoud的基本用法：给出建议、意见、批评

Should用来给出建议advice

You should buy the green one. It suits you.

You shouldn’t go to bed so late. It’s bad for you.

Should用来发表个人意见opinion

Everyone should see that film. It’s amazing.

The government should raise taxes on the rich.

Should用来给出批评criticism

You shouldn’t have said that.

He should have studied harder for his exams.

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Ought to

Ought to和should可以互换，但ought to更加formal and old-fashioned，在现代英语口语中很少见，可能会出现在正式的书面语中。

should的否定形式是shouldn’t

Ought to的否定形式是ought not to

在疑问句中，不用ought to，用should。

What time should I get there?

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be supposed to

比较以下两个句子：

I should be there by 9:00. 自己认为应该9点到

I am supposed to be there at 9:00. 本质是被动句，强调别人认为你应该9:00到(可能老板定了9点开会)，但你可能根本不care

.

再比较以下两个句子(婚礼背景)：

You should have worn something more formal. 说话人认为你应该穿的更正式点，有点指责你。

You were supposed to wear something more formal. 说话人可能并不care你穿怎样，但他提醒你婚礼场合或婚礼上的其他人希望参加者穿的正式一些。

Had better

比较以下两个句子：

You should finish everything today.

You’d better finish everything today.

had better含有警告的意味，经常会跟or，提醒/警告不按照说的做，会有不好的结果。You’d better be on time, or you could lose your job.

如果没有or分句，听话者也能听出其中的提醒，警告，威胁等等语气。

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