1. Only use MUST in the present tense. For other tenses use HAVE TO
OBLIGATION – MUST and HAVE TO
Only use MUST in the present tense. For other tenses use HAVE TO
I’d like to go camping with you, but I’ll have to ask my parents first.
In order to escape from the guards they had to swim across a river.
Use MUST when the obligation is something you agree with
Teacher to students: You must hand in your homework on Monday.
Use HAVE TO when the obligation comes from someone else
My teacher has given me a lot of homework which I have to do for Monday.
Use MUST for strong advice
You must be careful if you stay out late at night.

2. The past of SHOULD is SHOULD HAVE
Use BE SUPPOSED TO to talk about an obligation which may be different from what really happens
We’re supposed to do five writing tasks each term (but most people only do two or three).
Aren’t you supposed to be in class (not out here playing football)?
Use SHOULD to talk about the right thing to do, but which is different from what really happens
I should do the housework instead of watching television in the middle of the afternoon.
He should write his own answers instead of copying them from the internet.
The past of SHOULD is SHOULD HAVE
You shouldn’t have shouted at your father like that!

3. You can’t go in there – it says ‘No entry!’
PROHIBITION
Use these modal verbs and phrases to express prohibition: can’t, mustn’t, not let, am not allowed to, don’t allow (me) to
You can’t go in there – it says ‘No entry!’
You mustn’t speak during the exam – it is forbidden.
My sister won’t let me listen to her CDs.
I’m not allowed to use the kitchen in my host family’s house.
My parents didn’t allow me to play computer games when I was small.
Do not use DON’T HAVE TO to express prohibition
You mustn’t use your mobile phone in class (it’s not allowed).
Compare this with:
You don’t have to use your mobile phone to speak to Fayed. Look! He’s over there (i.e. it is not necessary).

4. I couldn’t leave the room until the end of the meeting.
To talk about the past use: couldn’t, didn’t let, wasn’t allowed to, didn’t allow (me) to
I couldn’t leave the room until the end of the meeting.
She wasn’t allowed to invite her boyfriend to the party.
Do not use MUSTN’T to talk about the past
We mustn’t couldn’t use our dictionary in the exam last week.

5. You can only smoke in open spaces, not inside buildings.
PERMISSION
To express permission use: can (past could), let, am allowed to and may (past was/were allowed to)
You can only smoke in open spaces, not inside buildings.
Are we allowed to use the phone in the office for private calls?
She let him borrow her bicycle to get to the station.
Only use MAY in formal situations
It is not necessary to stay till the end of the examination. When you have answered all the questions, you may leave the room.

6. NO OBLIGATION
To say that there is no obligation, or it’s not necessary use: don’t have to, don’t need to and needn’t
This is a really good exercise for phrasal verbs for anyone who’s interested, but it is not for homework, so you don’t have to do it if you don’t want to.
You needn’t learn all the vocabulary on this page – only the words you think are useful.
I DIDN’T NEED TO means it wasn’t necessary and I didn’t do it; I NEEDN’T HAVE means it wasn’t necessary but I did it
I didn’t need to buy a newspaper to find out the story because I’d already heard it on the radio.
What lovely roses! You needn’t have bought me so many, but it was very generous of you.