nurul septia

7. Modal auxiliaries
What are "modal auxiliary verbs"?

The verbs can, could, will, would, should, may, might, must, ought and shall are verbs which 'help' other verbs to express a meaning: it is important to realise that these "modal verbs" have no meaning by themselves. A modal verb such as would has several varying functions; it can be used, for example, to help verbs express ideas about the past, the present and the future. It is therefore wrong to simply believe that "would is the past of will": it is many other things.

A few basic grammatical rules applying to modal verbs

Modal verbs are NEVER used with other auxiliary verbs such as do, does, did etc. The negative is formed simply by adding "not" after the verb; questions are formed by inversion of the verb and subject:

You should not do that.
Could you pick me up when I've finished?
Modal verbs NEVER change form: you can never add an "-s" or "-ed", for example.
Modal verbs are NEVER followed by to, with the exception of ought to.
What sort of meanings do modals give to other verbs?
The meaning are usually connected with ideas of DOUBT, CERTAINTY, POSSIBILITY and PROBABILITY, OBLIGATION and PERMISSION (or lack of these). You will see that they are not used to talk about things that definitely exist, or events that definitely happened. These meanings are sometimes divided into two groups:
DEGREES OF CERTAINTY: certainty; probability; possibility; impossibility
OBLIGATION/FREEDOM TO ACT: permission,lack of permission; ability; obligation.
Let's look at each modal verb separately, and the functions they help to express:
WILL
Making personal predictions

I don't think the Queen will ever abdicate.
I doubt if I'll stay here much longer.
Talking about the present with certainty (making deductions)

I'm sure you will understand that there is nothing the Department can do
There's a letter for you. It'll be from the bank: they said they'd be writing.
Talking about the future with certainty

I won't be in the office until 11; I've got a meeting.
Don't bother ringing: they'll have left for their 10 o'clock lecture.
Talking about the past with certainty

I'm sure you will have noticed that attendance has fallen sharply.
Reassuring someone

Don't worry! You'll settle down quickly, I'm sure.
It'll be all right! You won't have to speak by yourself.
Making a decision

For the main course I'll have grilled tuna.
I'm very tired. I think I'll stay at home tonight.
Making a semi-formal request

Will you open the window, please? It's very hot in here.
Sign this, will you?
Offering to do something

You stay there! I'll fetch the drinks.
Insistence; habitual behaviour

I'm not surprised you don't know what to do! You will keep talking in class.
Damn! My car won't start. I'll have to call the garage.
Making a promise or a threat

You can count on me! I'll be there at 8 o'clock sharp.
If you don't finish your dinner off, you'll go straight to bed! 
SHALL
Shall is a form of will, used mostly in the first person. Its use, however, is decreasing, and in any case in spoken English it would be contracted to "-ll" and be indistinguishable from will.

The only time you do need to use it is in questions, when:
Making offers

Shall I fetch you another glass of wine?
Making suggestions

Shall we go to the cinema tonight?

MAY & MIGHT
May & might sometimes have virtually the same meaning; they are used to talk about possibilities in the past, present or future. ("Could" is also sometimes used).

May is sometimes a little bit "more sure" (50% chance); whereas might expresses more doubt (maybe only a 30% chance).

May & might are used, then, for:
Talking about the present or future with uncertainty

She may be back in her office: the lecture finished ten minutes ago.
I may go shopping tonight, I haven't decided yet.
England might win the World Cup, you never know.
Talking about the past with uncertainty

I'm surprised he failed. I suppose he might have been ill on the day of the exam.

They can also sometimes be used for talking about permission, but usually only in formal situations. Instead of saying May I open a window? we would say Is it all right/OK if I open a window? or Can I open a window? for example. You might, however, see:

Students may not borrow equipment without written permission.

MAY
Talking about things that can happen in certain situations

If the monitors are used in poorly lit places, some users may experience headaches.
Each nurse may be responsible for up to twenty patients.
With a similar meaning to although

The experiment may have been a success, but there is still a lot of work to be done. (= Although it was a success, there is still ...)
MIGHT
Saying that something was possible, but did not actually happen

You saw me standing at the bus stop! You might have stopped and given me a lift!
WOULD
As the past of will, for example in indirect speech

"The next meeting will be in a month's time" becomes
He said the next meeting would be in a month's time.
Polite requests and offers (a 'softer' form of will)

Would you like another cup of tea?
Would you give me a ring after lunch?
I'd like the roast duck, please.
In conditionals, to indicate 'distance from reality': imagined, unreal, impossible situations

If I ruled the world, every day would be the first day of Spring.
It would have been better if you'd word processed your assignment.After 'wish', to show regret or irritation over someone (or something's) refusal or insistence on doing something (present or future)

I wish you wouldn't keep interrupting me.
I wish it would snow.

(This is a complicated area! Check in a good grammar book for full details!)
Talking about past habits (similiar meaning to used to)

When I was small, we would always visit relatives on Christmas Day.
Future in the past

The assassination would become one of the key events of the century.
CAN & COULD
Talking about ability

Can you speak Mandarin? (present)
She could play the piano when she was five. (past)
Making requests

Can you give me a ring at about 10?
Could you speak up a bit please? (slightly more formal, polite or 'softer')
Asking permission

Can I ask you a question?
Could I ask you a personal question? (more formal, polite or indirect)
Reported speech

Could is used as the past of can.

He asked me if I could pick him up after work.
General possibility

You can drive when you're 17. (present)
Women couldn't vote until just after the First World War.
Choice and opportunities

If you want some help with your writing, you can come to classes, or you can get some 1:1 help.
We could go to Stratford tomorrow, but the forecast's not brilliant. (less definite)
Future probability

Could (NOT can) is sometimes used in the same way as might or may, often indicating something less definite.

When I leave university I might travel around a bit, I might do an MA or I suppose I could even get a job.
Present possibility

I think you could be right you know. (NOT can)
That can't be the right answer, it just doesn't make sense.

Past possibility

If I'd known the lecture had been cancelled, I could have stayed in bed longer.
MUST
Examples here refer to British English; there is some variation in American English.
Necessity and obligation
Must is often used to indicate 'personal' obligation; what you think you yourself or other people/things must do. If the obligation comes from outside (eg a rule or law), then have to is often (but not always) preferred:

I really must get some exercise.
People must try to be more tolerant of each other.
You musn't look - promise?

If you own a car, you have to pay an annual road tax.
Strong advice and invitations

I think you really must make more of an effort.
You must go and see the film - it's brilliant.
You must come and see me next time you're in town.
Saying you think something is certain

This must be the place - there's a white car parked outside.
You must be mad.
What a suntan! You must have had great weather.

The negative is expressed by can't:

You're going to sell your guitar! You can't be serious!
She didn't wave - she can't have seen me.
SHOULD
Giving advice

I think you should go for the Alfa rather than the Audi.
You shouldn't be drinking if you're on antibiotics.
You shouldn't have ordered that chocolate dessert - you're not going to finish it.

Obligation: weak form of must

The university should provide more sports facilities.
The equipment should be inspected regularly.

Deduction

The letter should get to you tomorrow - I posted it first class.
Things which didn't or may/may not have happened

I should have renewed my TV licence last month, but I forgot.
You shouldn't have spent so much time on that first question.
Ought to

Ought to usually has the same meaning as should, particularly in affirmative statements in the present:

You should/ought to get your hair cut.

Should is much more common (and easier to say!), so if you're not sure, use should.