Have to and Must

My experience as a teacher of English as a foreign language has taught me that how to use ‘have to’ and ‘must’ causes quite a few problems, so here is a quick guide on the subject. I usually break it down as follows:

Use ‘have to’ for outside forces:
have to go and visit the doctor, I have really bad mouse arm and it’s keeping me awake at night.
If I am to get to work on time, I have to set out at eight o’clock.
I only use my car when I have to, usually when I have to teach in another area.
have to wear smart clothes at work, it’s company policy.
It’s raining so hard that I’ll have to take the bus.

Use ‘must’ for inside forces, like your own feelings:
I really must lose weight! I look like a pear.
must rush. I’d like to get to work early so that I can leave early too.
must use my bike more and leave the car at home. I don’t get enough exercise.
must buy some new shirts; these are looking rather shabby.

The opposite of ‘have to’ is ‘don’t have to’:
don’t have to get up early tomorrow, I got the morning off.
don’t have to shave at the weekends, because I don’t have to work.
According to the tax office, we don’t have to charge VAT on handicrafts and second-hand goods.
You don’t have to spend lots of money to have a relaxing holiday.

‘Must’ does not have an opposite, so use ‘don’t have to’ instead.

‘Must not’ (mustn’t) has a different meaning, similar to ‘cannot’ (can’t), is usually quite strong and refers to rules and regulations etc:
You must not carry sharp objects or large amounts of liquid in your carry-on luggage.
Bicycle helmets must not be worn when visiting the bank.
You must not take photos here, it is a top security area.
You must not buy or sell pirated videos.

So, in summary:
have to lose weight, the doctor said so.
must lose weight, I feel unattractive.
don’t have to lose weight because I’m underweight already.
mustn’t lose weight – the European Union has standardised the human body!

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