Modal Auxiliaries

Modal auxiliaries are helping verbs used before the main verb to express ideas such as ability, permission, possibility, obligation, prohibition, advice, certainty, and lack of necessity.

Common modal auxiliaries:
must | must not | can | could | may | might | need not | should | ought to | had better

A modal auxiliary is always followed by the base form of the verb.

Correct: I can swim.
Incorrect: I can swims.

Correct: You must stop.
Incorrect: You must to stop.

Correct: He should revise.
Incorrect: He should revises.

Modal auxiliaries do not change according to the subject.

Correct: He can dance.
Incorrect: He cans dance.

Correct: She may come.
Incorrect: She mays come.

Must

Must is used mainly for strong obligation and logical conclusion.

Strong Obligation
Must is used when something is necessary, compulsory, or very important.

E.g. You must stop when the traffic lights turn red.
This means that stopping at a red traffic light is not a choice. It is a rule that must be followed.

Students must wear uniforms.
You must obey traffic rules.
We must respect our parents.

Logical Conclusion / Certainty
Must is used when one is almost certain about something because one has a clear reason.

E.g. He must be very tired. He has been working all day long.
This means the speaker is sure he is tired because he worked all day.

She must be hungry. She has not eaten since morning.
They must be at home. Their car is outside.

Must Not

Must not is used to express prohibition. It means something is not allowed.

E.g. You must not smoke in the hospital.
This means smoking in the hospital is strictly forbidden.

You must not use mobile phones during the exam.
Children must not play on the road.
We must not waste water.

Important Difference between Must Not and Need Not
Must not means something is forbidden. Need not means something is not necessary.

E.g. You must not go there.
Meaning: You are not allowed to go there.

You need not go there.
Meaning: It is not necessary for you to go there.

Can

Can is used for ability, permission, and possibility.

Ability
Can is used to show that someone is able to do something.

E.g. I can swim.
This means I have the ability to swim.

She can sing well.
He can speak English.
They can solve the problem.

Permission
Can is used to ask for or give permission in informal situations.

E.g. Can I use your phone, please?
This means the speaker is asking permission to use someone’s phone.

Can I come in?
Can we go out to play?
You can sit here.

Possibility
Can is used to show that something is possible.

E.g. Smoking can cause cancer.
This means smoking has the possibility of causing cancer.

Too much rain can cause floods.
Careless driving can cause accidents.
Hard work can bring success.

Could

Could is used for ability in the past, polite permission, and possibility.

Ability in the Past
Could is used to talk about an ability someone had in the past.

E.g. When I was younger, I could run fast.
This means the speaker had the ability to run fast in the past.

When she was a child, she could sing beautifully.
My grandfather could walk long distances when he was young.

Polite Permission
Could is more polite than can when asking permission.

E.g. Excuse me, could I just say something?
This is a polite way of asking permission to speak.

Could I borrow your pen?
Could I open the window?
Could we leave early today?

Possibility
Could is used when something is possible, but not certain.

E.g. It could rain tomorrow.
This means rain is possible tomorrow.

He could be late.
The match could be cancelled.
This answer could be correct.

May

May is used for permission, possibility, and probability.

Permission
May is used to ask for or give permission in a formal and polite way.

E.g. May I use your phone, please?
This is more formal than “Can I use your phone?”

May I come in?
May I go to the library?
You may leave now.

Possibility / Probability
May is used when something is possible.

E.g. It may rain tomorrow.
This means there is a chance of rain tomorrow.

She may come today.
The train may be late.
He may win the prize.

Might

Might is used for polite permission, possibility, and probability. It usually shows a weaker possibility than may.

Polite Permission
E.g. Might I suggest an idea?
This is a very polite way to ask permission to make a suggestion.

Might I ask a question?
Might I speak to you for a moment?

Possibility / Probability
E.g. I might go on holiday to Australia next year.
This means the speaker is thinking about going, but it is not certain.

It might rain in the evening.
She might join us later.
He might forget the appointment.

Difference between May and Might
May shows a stronger possibility. Might shows a weaker possibility.

E.g. It may rain tomorrow.
This sounds fairly possible.

It might rain tomorrow.
This sounds less certain.

Need Not

Need not means lack of necessity or absence of obligation. It means something is not necessary.

E.g. I need not buy tomatoes. There are plenty of tomatoes in the fridge.
This means buying tomatoes is unnecessary.

You need not bring your book today.
We need not wait any longer.
She need not worry about the test.

Important Point
Need not does not mean prohibition. It only means there is no need to do something.

Should / Ought To

Should and ought to are used for advice, mild obligation, and logical conclusion.

Advice
Should or ought to is used to advise someone.

E.g. You should revise your lessons. / You ought to revise your lessons.
Both sentences mean that revising lessons is a good idea.

You should eat healthy food.
You ought to respect your teachers.
He should sleep early.

Mild Obligation / 50% Obligation
Should / ought to can express 50% obligation. This means the obligation is not as strong as must.

E.g. I should see a doctor. I have a terrible headache.
This means seeing a doctor is advisable, but not mandatory.

You should complete your homework.
We ought to help the poor.
She should be careful while crossing the road.

Logical Conclusion
Should / ought to is used when one expects something to be true.

E.g. He should be very tired. He has been working all day long.
This means the speaker expects that he is tired.

The bus should arrive soon.
She ought to be at school now.
The result should be announced today.

Had Better

Had better is used to give strong advice. It often suggests that something bad may happen if the advice is not followed.

E.g. You had better revise your lessons.
This means the person should revise seriously. Otherwise, there may be a bad result, such as failing the exam.

You had better leave now, or you will miss the bus.
You had better take an umbrella. It may rain.
He had better apologise.

Difference between Should and Had Better
Should gives normal advice. Had better gives stronger advice, often with a warning.

E.g. You should study.
This is simple advice.

You had better study.
This sounds stronger and more urgent.

Summary

Modal auxiliaries help express different meanings clearly. Must shows strong obligation or certainty. Must not shows prohibition. Can shows ability, permission, and possibility. Could shows past ability, polite permission, and possibility. May and might show permission and possibility. Need not shows absence of necessity. Should and ought to show advice, mild obligation, and logical conclusion. Had better gives strong advice with a warning. Learning modal auxiliaries helps to speak and write English more correctly and meaningfully.