MUST / HAVE TO / NEED : OBLIGATION

MUST/HAVE TO/NEED

OBLIGATION

FUTURE OBLIGATION

MUST
WILL HAVE TO

FUTURE LACK/ ABSENCE OF OBLIGATION

NEED NOT
WON'T HAVE TO
WON'T NEED TO

FUTURE NEGATIVE OBLIGATION/ PROHIBITION

MUST NOT

PRESENT OBLIGATION

MUST
HAVE TO

PRESENT LACK/ ABSENCE OF OBLIGATION

NEED NOT
DON'T HAVE TO
DON'T NEED TO

PRESENT NEGATIVE OBLIGATION/ PROHIBITION

MUST NOT

PAST OBLIGATION

HAD TO

PAST LACK/ ABSENCE OF OBLIGATION

DIDN'T HAVE TO
DIDN'T NEED TO
HADN'T (GOT) TO

1. Must, must not and need not compared to the other forms.

a. Must, must not and need not express the speaker's authority:
You must do your homework before you watch TV.
You must not turn the TV till you have done your homework.
You needn't do your homework tonight. You can leave it till tomorrow.

b. The other forms have to/will have to/had to and won't/don't/didn't need to express external obligation:
KAMAL'S SISTER: KAMAL is starting work next week. He'll have to get up early. He'll hate that.
Sometimes MUST and NEED NOT can be used for external obligation also. NEED NOT is quite often used in this way, especially in the first person.

2. Must not and need not compared.

a. MUST NOT expresses negative obligation or the speaker's emphatic advice ( prohibition ):
You must not tell anyone.
ZOO NOTICE: Visitors mustn't feed the giraffes.
RAILWAY NOTICE: Passengers must not walk on the railway line.

a. Obligation.

AFFIRMATIVE

Must can be used to give strong advice or orders to oneself or other people.
I really must stop smoking.
You must be here by eight o'clock at the latest.
When must is used the obligation comes from the speaker. If the obligation comes from
outside must is possible but HAVE TO is more common.
I have to work from 9.00 a.m. till 5.00 p.m.

INTERROGATIVE

In questions, must is used to ask about the wishes or intentions of the person one is speaking to.
Must I clean all the rooms?
Why must you always leave your dirty clothes in the bathroom?

NEGATIVE

In negative sentences, DON'T NEED TO, NEEDN'T or DON'T HAVE TO, is used to say
that there is no obligation; MUSTN'T is used to tell people no to do things:
You needn't work tomorrow if you don't want to.
You mustn't move any of the papers on my desk.
Must can only be used to refer to present and future obligation. To talk about the past, had to is used.
I had to leave early because I wasn't feeling well.

b. Deduction.

Must can be used to say that we are sure about something (because it is logically necessary)
Maryam must have some problem: she keeps crying.
I'm in love. That must be nice.
That's the doorbell. It must be Rachid.
Must is only used in this way in affirmative sentences. In questions and negatives we use can or can't instead.
That can't be the postman. It's only seven o'clock.
What do you think this letter can mean?
Must is used with the perfect infinitive for deductions about the past. (can and can't for questions and negatives)
We went to Zagora. That must have been nice.
The lights have gone out. A fuse must have blown.
I don't think he can have heard you. Call again.
Where can Jalal have put the matches?
He can't have thrown them away.
In reported speech, must can be used after a past reporting verb as if it were a past tense.
(Only in that case, must refers to the past).
I decided that I must stop smoking.
I felt there must be something wrong.

HAVE TO

Have to is used, with a following infinitive, to express the idea of obligation.
How often do you have to travel on business?
Sorry I have to go now.
We make a distinction between habitual or repeated obligation, and non-habitual obligation. When there is the idea of repetition we use ordinary verb-forms, with do in questions and negatives.
I don't usually have to work on Sundays.
Do you often have to speak French in your job?
When we are talking about one thing that we are obliged to do, it is more usual to use got-forms.
I haven't got to work tomorrow.
Have you got to do any interpreting this week?
Got-forms are unusual in the past, and are replaced by ordinary verb-forms of infinitive and participles.
Did you have to go to the kindergarten when you were a child?
To talk about the future, both have to and will have to are common.
I've got to get up early tomorrow. We're going to Fes.

HAVE TO AND MUST

1. Both of these verbs are used to talk about obligation.
Must is most often used to talk about an obligation that depends on the person speaking or listening: if I say that you or I must do something, I probably mean that I feel it is necessary.
Have to is generally used to talk about obligations that come from "outside".
I must stop smoking (I want to).
You must try to get to work on time (I want you to)
I must make an appointment with the dentist (I've got toothache)
This is an awful party, we really must go (I want us to go).
You've got to go and see the boss (he wants you to).
Muslims have to go to the Mosque on Fridays (our religion tells so)
I've got to see the dentist tomorrow (I have an appointment).
This is a lovely party, but we've got to go because of the baby-sitter.

2. Must, in questions, asks about the wishes of the person one is speaking to.
Do your homework. Oh, must I?

3. Must has no past form; past obligation is usually expressed by using had to (except in
reported speech)
When I was your age I had to get at 5 every morning.
I told him he mustn't make a decision.

4. The negative forms mustn't and don't have to have quite different meanings. Compare:
You mustn't tell Badr. (Don't tell him)
You don't have to tell Badr. (You can if you want but it isn't necessary)

5. Instead of don't have to and haven't got to, needn't is often used in British English.
You needn't tell Badr.

NEED.

a. The ordinary forms of need are much more common than the modal auxiliary forms. The only modal form which is often used is needn't.
You needn't try to explain.
Do you need to stay this evening?
When the modal forms are used, they usually refer to immediate necessity; they are often used to ask for or give permission -usually permission not to do something. Ordinary verb forms are more common when we talk about habitual, "general" necessity. Compare:
(1) We needn't book a table. The restaurant won't be full.
Need I do the washing up? I'm in a hurry.
(2) Do you need to get a visa if you go to Algeria?

b. Present tense forms of need can be used to talk about the future, but will need to is often used to give advice. Compare:
(1) Need I come in early tomorrow? (Or, Do I need to come in...)
I need to get the car service soon.
(2) You'll need to start work soon if you want to pass your exams.

c. Affirmative modal forms are possible after negative verbs, and in sentences which express doubt or negative ideas.
I wonder if we need take sleeping-bags.
I don't think he need go just yet.
The only thing you need do is fill in this form.
(You don't need to do anything else)
Note that these affirmative modal forms are mainly used in a formal style. In informal usage we would probably use the ordinary forms.
I wonder if we need to take sleeping-bags.
I don't think he needs to go just yet.
The only thing you need to do is fill in this form.

d. Note the difference between needn't and mustn't. Needn't is used to say that there is no obligation; mustn't expresses an obligation not to do something because it is forbidden. Compare:
You needn't tell Salma. She already knows.
You mustn't tell Salma. I don't want her to know.

NEEDN'T + perfect infinitive.

If you say that somebody needn't have done something, it means that he did, but that it was unnecessary:
You needn't have woken me up: I don't have to go to work today.
I needn't have bought all that coke. Only three people came to the party.
The ordinary past (didn't need to) is not quite the same. Compare:
She needn't have hurried (It wasn't necessary but she did)
She didn't need to hurry (It wasn't necessary. We don't know if she did).