英文文法　ｓｈｏｕｌｄ　ｏｕｇｈｔ　ｔｏ　ｈａｖｅ　ｔｏ　ｍｕｓｔ

-- Must (必須,得)
You can use MUST to say that you believe something is certain.

Example:
You've been travelling all day. You must be tired. (Travelling is tireing and you've been travelling all day, so you must be tired)

-- Have to (必須)
I HAVE TO do something = it is necessary to do it, I am obliged to do it.

Example:
You can't turn right here. You have to turn left.

-- Ought to ((表示義務,責任等)應當,應該)
You can use OUGHT TO instead of should in the sentences.

Eample:
Do you think I ought to apply for this job? (Do you think I should apply this job?)

should, ought to, have to, must 既意思差不多,都是解應該
但他們的層次不同

最弱
should
ought to
have to
must
最強