The modal can is used,
1. To express ability, temporary or permanent,
• He can speak four language including French.
• The doctor can see you only in the afternoon.
2. To express permission,
• You can go home now.
• He can not touch the paintings.
3. To express possibilities,
• The accident can happen at any time.
The modal may is used
1. To express permission more in formal way than can.
• May I borrow your car?
• You may collect your notebook.
2. To express possibilities,
• It may rain today.
• He may not get a good job.
3. To express a wish,
• May he live long!
The modal could is used
1. As the past tense of can in indirect sentence,
• He said, " he can finish the book in a day".
• He said that he could finish the book in a day.
2. To express ability in the past,
• I could read without glasses six months ago.
• We could buy a good meal for fifty paise twenty years ago.
3. To express polite request,
• Could you wait for a second?
• Could you tell me the time?
4. To express contingent possibilities, i.e. the possibilities of something happening under some conditions,
• If we had the money, we could buy that house.
• You could get first class if you made up your mind to work hard.
The modal might is used,
1. In the past tense of may in indirect sentence,
• He said, "Result may appear in the next day's paper".
• He said that result might appear in the next day's paper.
2. To express possibilities but in a lesser degree than ma,
• It might rain.
• The Chief Minister might visit our town.
3. In questions might implies more politeness than may,
• Might I have a word with you?
• Might I make a suggestion?
The modal must is used,
1. To express obligation, compulsion, necessity, or emphatic advice. It means have to, have got to,
• The letter must leave by today's mail.
• You must see a doctor at once.
2. In reported speech must is used instead of had to,
• The teacher said that the boys must bring their books to the class everyday.
3. In negative mustn't means not permitted to,
• You mustn't walk on the grass.
• You mustn't use my vehicle.
4. To express logical necessity or expectations, or something that can be inferred from circumstances,
• If this is litmus paper, it must turn blue.
• There must be a mistake somewhere.
Note: Can is used instead of must in case of negative or interrogative sentences,
• There can't be a mistake anywhere.
• Can this be true.
The moral need is commonly used,
1. In statement with not or in questions without not, need not expresses the idea that there is no compulsation,
• A: Must I leave tomorrow?
• B: No, you needn't.
Note: The following examples shows where to use must, need not and must not.
You must eat all of it.
You must see him today.
You must wait.
• You needn't eat all of it.
• You needn't see him today.
• You needn't wait.
• You mustn't eat all of it.
• You mustn't see him today.
• You mustn't wait.
2. In interrogative sentences,
• Need I wait any longer?
• Need I tell you anything more?
3. As a moral doesn't have a past tense. But as a full verb it takes the past tense form,
• I needed a hundred rupees for expenses, but I had only fifty.
The moral ought is used to express duty, or moral or social obligation. It is used nearly the same way as should is used.
• You ought to obey your parents.
• You ought not to speak ill about others.
• Ought you to go today?
• You ought to be ashamed of yourself for such conduct.
Note: The verb that follows ought always take the to-form.
The moral dare is used
1. To express fearlessness of any consequences,
• How dare you say such a thing.
• Dare we interrupt him.
2. As an ordinary verb to mean have courage,
• He did not dare to criticize the government.
The moral used to is used
1. Used to + verbs expresses a habit in the past,
• He used to tell lie a lot.
• He used to go for walk every morning in past.
Note: Habit in the present can be expressed as a present simple form.
2. To express the existence of something in the past,
• There used to be a park here long time ago.
3. In interrogative sentence,
• Used he to smoke?
• You have to get used to standing in a queue theses days.
4. To form negative sentence to express the habit,
• He used not to go for films, but now he goes often.
5. Note: An alternative form,
• He didn't use to smoke, but now he does.
That could have been Fred.
That might have been Fred.
She has to pay her own tuition at college.
He has to have been the first student to try that.
Might I leave the room? (