Modal Auxiliary

What are modal verbs? 

Modals (also called modal verbs, modal auxiliary verbs, modal auxiliaries) are special verbs which behave irregularly in English. They are different from normal verbs like "work, play, visit..." They give additional information about the function of the main verb that follows it. They have a great variety of communicative functions.

Here are some characteristics of modal verbs:

• They never change their form. You can't add "s", "ed", "ing"...
• They are always followed by an infinitive without "to" (e.i. the bare infinitive.)
• They are used to indicate modality allow speakers to express certainty, possibility, willingness, obligation, necessity, ability

List of modal verbs

Here is a list of modal verbs:

can, could, may, might, will, would, shall, should, must

The verbs or expressions dare, ought to, had better, and need not behave like modal auxiliaries to a large extent and my be added to the above list

Use of modal verbs:

Modal verbs are used to express functions such as:

1. Permission
2. Ability
3. Obligation
4. Prohibition
5. Lack of necessity
6. Advice
7. possibility
8. probability

Examples of modal verbs

Here is a list of modals with examples:

Modal Verb	Expressing	Example
must	Strong obligation	You must stop when the traffic lights turn red.
	logical conclusion / Certainty	He must be very tired. He's been working all day long.
must not	prohibition	You must not smoke in the hospital.
can	ability	I can swim.
	permission	Can I use your phone please?
	possibility	Smoking can cause cancer.
could	ability in the past	When I was younger I could run fast.
	polite permission	Excuse me, could I just say something?
	possibility	It could rain tomorrow!
may	permission	May I use your phone please?
	possibility, probability	It may rain tomorrow!
might	polite permission	Might I suggest an idea?
	possibility, probability	I might go on holiday to Australia next year.
need not	lack of necessity/absence of obligation	I need not buy tomatoes. There are plenty of tomatoes in the fridge.
should/ought to	50 % obligation	I should / ought to see a doctor. I have a terrible headache.
	advice	You should / ought to revise your lessons
	logical conclusion	He should / ought to be very tired. He's been working all day long.
had better	advice	You 'd better revise your lessons

Remember

Modal verbs are followed by an infinitive without "to", also called the bare infinitive.

Examples:

• You must stop when the traffic lights turn red.
• You should see to the doctor.
• There are a lot of tomatoes in the fridge. You need not buy any.

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.

http://www.myenglishpages.com/site_php_files/grammar-lesson-modals.php

http://library.bcu.ac.uk/learner/Grammar%20Guides/3.07%20Modals.htm