Must with instructions and orders and the law

[Rachel Adams]

Hello.

This screenshot is from Macmillan English Grammar in Use. Could you please tell me if modal verbs 'have to' and 'must' are used interchangeably in its examples? In BrE and when tallking about the law, instructions, orders and when it is important to do something.

1. 'Children have to start school when they are five.' (It's the law).

2. 'I must write to Aunt Julia'. ( I think it is important).

3. 'You must do your homework more carefully.' (The speaker thinks it is important).

4. 'Visitors must report to reception on arrival.' (Instruction/order).

5. 'You must be back home by 12 o'clock.'

6. 'He has to do military service next year.' (Very important or necessary. Or they probably mean it's the law).

 

[Rachel Adams]

As I said to you in a very recent thread, 'must' suggests to me an an obligation imposed by the speaker, 'have to' an externally imposed obligation. The Macmillan text expresses a similar thought.

That being so, 'must' and 'have to' are not interchangeable in those sentences, except in #5, which could be the speaker's rule or an externally imposed one.

In the first sentence it is imposed by the law so only 'have to works'. But in #4 if I use 'have to' to imply that the obligation comes from the staff it won't be wrong either. Right?

 

[Rachel Adams]

In #1, 'must' is possible if the words are said by education minister.
In #4, 'have to' is possible if the obligation comes from somebody else.

However, this does not mean that 'must' and 'have to' are interchangeable in those sentences as you presented them. You have changed the context.

I see. Thank you much! In #6 'must' is possible if it said by a military official. Right?