Explanation of HOW to express OBLIGATION & NECESSITY

MUST / HAVE TO

OBLIGATION / NECESSITY

HAVE TO

We use “have to” when the necessity comes from outside the speaker or when others decide for the speaker.

Check these examples:

• All the primary school students in Turkey have to wear a blue unifrom.
• All the men in Turkey who are 18 years old or older have to do their military service.
• Cenk has to wear a suit at work. ( The company’s board decided )
• Before entering the factory, all the workers have to sign the attendance book.
• As it is an international company Murat has to work on Saturdays to deal with the customers.

All of the necessities in the examples above come from outside the speaker.

Negative form of “have to”: DON'T/ DOESN'T HAVE TO

We use 'don't/ doesn't have to'  to state that there is NO obligation or necessity.

• You don't have to come if you don't want to.
• He doesn't have to sign anything if he doesn't want to at this stage.

•  University students  don't have to attend lessons every week day.
• Men in Argentina who are 18 years old or older don't have to do their military service.
• Celia doesn't have to arrive at work at 9. She has flexitime. 

Past form of “have to” is “had to”/ "didn't have to"

Check these examples:

• When I was in the army I had to shave everymorning.
• Linda had to fill an application form to be a candidate for the position in the company.
• When I was a child, I had to go to bed at 9p.m.
• In my previous job all the ladies had to wear a tie.

We use “will have to” for future necessities.

• Mrs.Hanks will have to work in summers in her new job.
• If Burak is promoted, he will have to check the reports everyday.

MUST

We use “must” for the necessities that the speaker decides himself. Must is used only in present and future.

• I cough a lot in the evenings. I must stop smoking. ( I decided myself )
• As Meral has a very important exam tomorrow morning she mustgo to bed early.
• This client is a very serious man so we must prepare everything very carefully.
• I must save money to buy that house.

Past form of “must” is “had to”.

• I had to get a taxi for work yesterday because I was late.
• In order to get to the office, I had to take three buses last year.
• She had to work last Saturday because she couldn’t finish the reports.

must not (mustn't)

must not is  used for something that is prohibited (not allowed)

"I mustn't smoke at work."

We use 'mustn't' to express strong obligations NOT to do something.

• We mustn't talk about it. It's confidential.
• I mustn't eat chocolate. It's bad for me.
• You mustn't phone me at work. We aren't allowed personal calls.
• They mustn't see us talking or they'll suspect something.