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Their mothers want them home by 8pm!

A question from Claudio from Switzerland:

'What is the difference between 'mustn't' and 'don't have to'?

mustn't / don't have to


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Susan Fearn:

Hi Claudio, well first of all that's a very good question - and confusing 'mustn't' and 'don't have to' is something that I find learners of English often do, perhaps because, from the point of view of meaning, the positive and negative forms of these verbs don't quite match.

'Must' and 'have to' are of course what we call 'modal verbs' and this category of verbs also includes 'may', 'might', 'should', 'can', 'could' and so on. There are two things to know about modal verbs:

Firstly, they usually follow a particular grammatical pattern - they're followed by the infinitive, usually without 'to', so: 'You must go', 'I might see him' and so on.

And secondly, it's not a bad idea to learn them according to their meaning or how they're used - their function, if you like. For example, they're used to show possibility or give permission. And this brings me back to 'must' and 'have to'.

Now, one of the ways that 'must' is used in the positive, is when a speaker's telling you to do something and you really don't have any choice - it's an obligation. And here's an example:
My daughter wants to go out and I say to her,
"Well, you can go out, but you must be back by 8."
I have the authority, I'm her mother.

I could have said:
"You have to be back by 8" - but that would have been a bit weak. The basic meaning would be the same - still obligation - but it would sound much less strong. Now that's because 'have to' shows obligation more generally - it doesn't necessarily come from a strong, outside authority. So that's the positive forms.

But when we come to the negative forms, the difference in meaning is bigger.
For example, if I say to my daughter:
"You mustn't stay out after 8" that's my authority, my rule. I'm forbidding her from staying out any later; she has no choice.

So 'must', 'mustn't' - there's a kind of strong authority there.

But if I say:
"You don't have to stay out after 8", it's much weaker - in fact it means something a little bit different. It implies she doesn't really want to go out and she doesn't really want to come back after eight and I'm reassuring her that that's okay; she's got a choice.

So, 'don't have to' means you don't need to, you're not obliged to, it's not necessary for you to stay out if you really don't feel like it - you have a choice.

Susan Fearn has taught English in Europe, Japan and China and has made programmes for 蜜芽传媒 Learning English in the past. She is currently teaching English for Journalism and Public Relations at the University of Westminster in London.

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