Must not

We say ‚must not‘ when there is an OBLIGATION NOT to do something. It is wrong to do something.

It is also possible to use ‚can’t‘.

Must not is very formal and is used in public notices, e.g. Passengers must not eat on the bus.

Examples:

You must not do it! = You can’t do it. (I strongly advice you NOT to do it)

You mustn’t smoke here! (it is forbidden to smoke here)

She mustn’t tell her parents about that. (I feel strongly that she shouldn’t tell them)

Customers must not take more than 3 items at the time into the fitting rooms. (notice in a shop)

For past and future tenses we use ‚have to‘

Need not

We use ‚ need not/ don’t need to‘ when there is NO OBLIGATION to do something.

Also, when we say ‚ needn’t, we emphasise the fact that it’s not necessary to do something.

Present tense

Speaker’s authority:

You needn’t tell me straight away. It can wait.

I needn’t go there tomorrow. I can go there some time next week.

External authority:

You don’t need to decide right now because the boss said it could wait a bit longer.

He doesn’t need to get up early tomorrow because it’s Saturday.

Past tense

We use only one form: didn’t need to .

Examples:

We were told we didn’t need to pack a sunscreen when going to Norway for our holiday.

He didn’t need to send me all the relevant details immediately but he did so. (it wasn’t urgent to do so)

We didn’t need to look for a hotel in London because we stayed with our friends there.

Future tense

Speaker’s authority:

We use ‚needn’t‘ form also for the future

Examples:

You needn’t go there next week. I’ll go there instead.

You needn’t turn off your mobile during the exam but you’ll have to hand it over to the examiner.

External authority:

We use ‚ won’t need to‘ when talking about future.

Examples:

You won’t need to come in and sign the contract, they’ll post it to you.

People won’t need to worry about the global warming if they reduce the output of CO2 dramatically very soon.