If you want to be more formal, you say you may ... .

	can	the most commonest and most informal
	could	more hesitant and polite; we use it when we are not sure permission will be granted
	may	more formal, polite, respectful
	might	more hesitant, polite and respectful and is rather less common

The commonest way of refusing someone permission is to use an expression such as sorry, I’m sorry, I’m afraid not or I’d rather you didn’t, and give an explanation.

You can watch TV as long as you like.  You can not watch TV as long as you like.  You may watch TV as long as you like.  You may not watch TV as long as you like.

can’t, may not (not couldn’t) or must not can be used to refuse permission.

– Could we picnic here? – I’m sorry. I am afraid you can’t.

Members may not bring more than two guests into the club.

Luggage must not be left unattained.

NOTE	Permission issuing from some other authority can be granted or withhold more emphatically with be allowed to, be permitted to, and be forbidden to.
	You can / cannot You mustn’t You are (not) allowed to You‘re (not) permitted to You’re forbidden to	smoke here. smoke here. smoke here. smoke here. smoke here.

7. Prohibition. cannot / can’t ‘нельзя + инфинитив’ is used to say that something is forbidden, esp. because of a rule or law.

Children cannot bathe except in the presence of two lifesavers.

We’re awfully sorry we can’t let you stay here.

may not is used in a similar way to cannot, but is more formal.

Communion may not be celebrated by one who is not ordained.

8. Suggestions, offers and requests.. The choice of a modal verb for suggestions, offers and requests depends on the formality of the situation. can ‘(не) можете ли вы ..; (не) могли бы вы ...’ is used to make a request in a simple and direct way. could is more polite that can. possibly can be added to show that the speaker is asking for something extra. Using can’t or couldn’t instead of can or could makes a request sound more persuasive.

Can I help you?

can / could is used in interrogative (general questions) and negative-interrogative sentences and is followed by the non-perfect infinitive.

Sometimes we use can / could I have ...? with the meaning ‘Can you get it for me?’

Can I have my suitcase? (= Can you get my suitcase for me?)

Requests are often longer when the situation is more formal, and / or there is a strong possibility of a negative response. couldn’t expresses the speaker’s hopes for a more favourite answer that has just been indicated.

I can’t wait.  Could you wait five minutes?

Can you give me your pen for a minute? (informal; likely to get a positive response)

There is a huge fashion, but I was wondering if you could lend me your music system for the party? (more formal; less likely for get a positive answer)

you couldn’t ..., could you? is used to express a not very hopeful answer (when the speaker doesn’t really expect a favourable answer).

Can I ask a question?

Could I just interrupt a minute?

Can I come with you?  Can’t I come with you?

Couldn’t we stay here?

Could you possibly lend me $ 500?

You couldn’t wait for five minutes, could you?

To make a suggestion could is used in a declarative sentence or couldn’t in an interrogative sentence.

You could have a nursery there.

Couldn’t you just build more factories?

Politeness. If you want to say something during a discussion you can indicate politely that you are going to say it by using can / could (polite).

Perhaps I can mention another possibility. Perhaps I could just ask you this.

offers	Can I help you? We could do that for you. (less formal) May I help you? (more formal) Might I be of some assistance? (more formal)
requests	Can you close the window? Could you pass me the salt? (less formal) You might get me some milk while you are there. (more formal)
suggestions	We can try that new cafe. You could lose some weight. (less formal) You might give John a ring. (more formal)

can / could always are used for an alternative or more tentative suggestion.

We could always go to the Chinese cafe.

In a declarative sentence could is used to make a suggestion.

We could have tea.

In a discussion, can suggests that you approve of the opinion or description. could is more neutral.

English can be seen as an expressive discipline.

You could call it a political offence.

NOTE	Here are some other expressions to make requests longer and more polite. Would it be possible to borrow your car tonight? Do you think you could help me? If we begin a request with a high intonation it sounds more polite.

9. When could is used to give an instruction or order, it is more polite that would.

Could you make out her bill, please?  Would you show me how to do this?

10. When could is used to ask for help, it is more polite than would.

Could you show me how to do this?

can with you can also be used to ask for help when you are not sure whether someone will be able to help you or not.

Oh hello. Can you help me? I’ve been trying to get a London number for tem minutes and I can’t get through.

11. Only could ‘мог бы’ + indefinite / perfect infinitive (Subjunctive) has sometimes the meaning of irritation, criticism or reproach for failing to do what is or was in one’s powers, i.e. for non-performance of an action.

You could help your sister now when she is in trouble.

You could have helped your sister when she was in trouble.

You could work harder, if you want to finish it.

You could have told me. (= I’m annoyed / disappointed that you didn’t tell me.) = You should have told me.

In the meaning of reproach could (Subjunctive) is followed by the non-perfect infinitive with reference to the present and by the perfect infinitive with reference to the past.

12. can + be + adj / noun has the effect of ‘is sometimes’ or ‘is often’ and refers to capacity / possibility. It can be replaced by be capable of + -ing, but not by be able to.

It can be cold in India. (=It’s sometimes – or often – cold.)

can / could are used to say that something or someone is capable of having a particular effect, or of behaving in a particular way.

It can be very unpleasant.

He could really frighten me.

13. can / could are also used to refer to a general characteristic or quality that may show itself from time to time.

A home in London can cost a lot of money.

14. could / may / might are used when we are making a deduction based on evidence or on our experience.

He always wears smart suits.  He could / may / might be a businessman.

Negative deductions about a present event can be expressed by can’t / couldn’t with the present infinitive of the verb be or with the continuous infinitive of any verb.

Child: Can I have some sweets? I’m hungry.

Mother: You can’t / couldn’t be hungry. You’ve just had dinner.

To express a negative deduction about the past can’t have or couldn’t have are used.

She can’t have fixed the computer, it’s still not working properly. (= I’m sure she didn’t fix it.)

couldn’t must be used when the deduction is made in the past or introduced by a verb in the past tense. Otherwise either can’t or couldn’t can be used.

She said I couldn’t have come on the Circle Line.

He said it couldn’t be an aeroplane.

NOTE	When we are less certain we use may not have or might not have. We’d better phone them, they might not have heard the news.

15. Strong assertion. could not with the comparative form of an adjective is used to say that it is not possible for someone or something to have more of a particular quality.

I couldn’t be happier.

He could hardly have felt more ashamed of himself.

16. can / could / be able to are used to describe arrangement for a time in the future.

The doctor could see you at six; he can’t see you before then as he’s busy.

I’ll be able to see you after classes.

NOTE	If the arrangement is uncertain we use may or might. The doctor might be free to see you immediately after lunch; I’ll have to check the diary.

17. Unwillingness. cannot / can’t is used to indicate that you have strong feelings which prevent you from doing something.

I cannot leave everything for him.

I can’t give you up.

couldn’t is used in 2 ways to express unwillingness to do something:

• to indicate that you are unwilling to do something because you are afraid, embarrassed, or disgusted;

I couldn’t let him touch me.

• to indicate that you are unwilling to do something because you think it would be unfair or morally wrong.

I couldn’t take your last cigarette.

could have can also express unwillingness.

We couldn’t have left the dog on its own. (= So we didn’t.)

18. Wishes. If the verb is an event verb, the reference is made to the future.

I wish I could come on holiday with you next year.

19. could in the sense of ‘would be able to’ is used to express unreal past. That is the dependent use of the modal verb.

I’m sure you could get into university. (= If you applied.)

could + never has the sense of ‘would never be able to’.

I could never put up with it!

20. Emotionally coloured can and could are used in special questions to express puzzlement, impatience, etc. Different forms of the infinitive are possible.

What can / could he mean? (Что он, собственно, имеет в виду?)

What can / could he have meant? (Что он, собственно, имел в виду?)

What can he be doing? What can he have done?

Where can he have gone to?

21. Set-phrases:

• I can’t help (doing) sth ‘не могу удержаться; не могу не делать’;

– Stop biting your nails. – I can’t help it.

• I can’t help feeling / thinking / wondering etc. sth;

I can’t help feeling that there has been a mistake.

I couldn’t help thinking about the past.

• I couldn’t help myself / she couldn’t help herelf etc. means ‘to be unable to stop yourself from doing something you should not do’;

Yet when talk turns to Linux, it’s as if McNealy can’t help himself. He knows he should be courting the world’s Linux devotees, but instead he pokes fun at them.

Mary knew she sounded just like her mother but she couldn’t help herself.

• It can be helped (spoken) is used to say that there is nothing you can do to change a bad situation;

Johnny said he had to leave Mary for a while, it couldn’t be helped.

I am sorry you won't let me look at the picture once again. But that can't be helped. I quite understand what you feel about it.” (O. Wilde. The Picture of Dorian Gray)

• not if I can help it (spoken) is used to say that you are not going to do something;

– Are you going to watch the school play? – Not if I can help it.

• I couldn’t (spoken) is used to politely say that you do not want any more food or drink;

– Would you like another cup of tea? – Oh, thanks, I couldn’t.

• couldn’t be better / worse / more pleasant, etc. is used to emphasise how good, bad, etc. something is;

– How are things? – Fine! Couldn’t be better!

• sb sould do worse (than) do sth (spoken) is used to say that you think that someone should do something;

He could do worse than marry her.

• I can’t but do sth ... ‘ничего другого не остается как ...’;

A visitor to Dalmatia’s crown jewel, Dubrovnik, can’t help but be impressed.

• I can but / I cannot but … ‘не могу не ..., остается только ...’;

Dear Sir, I cannot but confess the failures of my correspondence, but hope the same regard which you express for me on every other occasion, will incline you to forgive me. (Boswell. Life of Johnson)

• cannot hope but … ‘не могу не ...’; / cannot but hope ‘остается только ...’;

She had soon afterwards reason to believe that the beginning was already made, and could not but hope that the gipsy, though she had told no fortune, might be proved to have made Harriet's. (J. Austen. Emma)

• I can’t / couldn’t possibly do sth ... ‘просто не могу / мог сделать ...’

«You can't possibly do that,» said the Rose: «I should advise you to walk the other way.» (L. Carroll. Through the Looking-Glass and What Alice Found There)

In short, he plainly hinted that we could not possibly do better than try pot-luck at the Try Pots. (Melville. Moby-Dick or, The Whale)

• I cannot help but ... ;

Pray pardon me, I cannot help but smile... (J. Keats. Complete Poems)

His want of tact had deeply offended her – not by seeing what he could not help, but by letting her know that he had seen it. (T. Hardy. Far from the Madding Crowd)

• as can be ‘очень, чрезвычайно’;

She is as pleased as can be.

The neighbours were as generous as could be.

• couldn’t care less (spoken) is used to say that someone does not care at all about something;

I really couldn’t care less what you do!.

• couldn’t agree more means ‘to agree completely’;

– We have to talk. – absolutely. I couldn’t agree more.

• I could do with ... (spoken) means ‘to heed or want something’.

We could do with a new fridge. (= This is a less strong version of : ‘We need a new fridge.’)

I could do with a cup of hot tea.

• can’t very well (do sth) ‘не чувствовать себя вправе’ is used to say that you can’t do something because it would be unacceptable;

I can’t very well tell him we don’t want him at the party!

• can hardly ‘не могу же’;

I can easily suppose that his age may appear much greater to you than to my mother; but you can hardly deceive yourself as to his having the use of his limbs! (J. Austen. Sense and Sensibility)

• can (hardly / scarecely) believe sth is used when you are very surprised or shocked by something;

One day, in May 2001, something happened to me that thousands of young actors dream about. A Hollywood film script landed on my doormat. I couldn’t believe it – I was excited and flattered.

“I can’t still believe that I’m recognized in Japan,” she says. Before long, she may have fans in the rest of Asia as well.

• what can you expect? is used to say that you are not surprised by something unpleasant or disappointing;

He was late, but what cant you expect?

• how can you expect ...? is used to say that it is unreasonable to think that something will happen or be true;