Page 367 - Practical English Usage 3ed - Michael Swan, Oxford

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must and should
Should can be used as a weaker fonn of must (see 520). Compare:
Ann must be at home by now. (;;; I think she's certainly at home.)
Ann should be at home by now. (= I think she's very probably at home.)
must (3): necessity and obligation
The following explanations apply particularly to British English. Americans often use have (got) to where British people use must (see 361). However, this use of have (got) to is becoming more common in British English under American influence.
statements: I really must stop smoking
In affirmative statements, we can use must to say what is necessary, and to give strong advice and orders to ourselves or other people.
Plants must get enough light and water if they are to grow properly. British industry must improve its productivity.
I really must stop smoking.
You must be here before eight o'clock tomorrow.
Must is common in emphatic invitations. You really must come and see us soon.
questions:MustI...?
In questions, we use must to ask about what the hearer thinks is necessary. Must I clean all the rooms?
Why must you always leave the door open?
negatives: You mustn't/can't ...
We use must not / mustn't to say that it is wrong to do things, or to tell people not to do things. Can't is also possible.
The government mustn't/can't expect people to work for no money. You mustn't/can't open this parcel until Christmas Day.
mustn't and needn't / don't have to
Mustn't is not used to say that things are unnecessary. This idea is expressed by needn't, don't need to (see 366) or don't have to.
You needn't work tomorrow ifyoll don't want to. OR You don't have to work . .. (NOT YBft InftS~'t't • . . ifYBft liB,,'t ftftI,tt lB.)
You don't need to get a visa to go to Scotland. OR You don't have to get a visa . .. (NOT Yaft mltStn't get a tlisa ffl g8 ta Sc6tlanli.)
past necessity and obligation
Must is not normally used to talk about past obligation (except in indirect speech - see below). This is because must is used mainly to influence people's· behaviour - for example through orders or advice - and one cannot do this in the past. Had to is used to talk about obligation that existed in the past.
I had to cycle three miles to school when I was a child.
My parents had to work very hard to build up their business. •
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