Modals are the auxiliaries which express attitudes like permission, possibility, necessity etc., such as :
Can Could May Might Shall Should
Will Would Must Ought Need Dare
They are also called Modal Auxiliaries.
Uses of Modals
Can is a Principal verb followed by an Infinitive with its to omitted. Its Past Tense is could but, it has no Past Participle. It means ability or capacity.
I can help you.
I can swim across the river.
May is used in expressing doubt or in asking or giving permission.
He may catch the train (doubt).
May I go out? (asking permission).
You may sleep now (permission).
May is used to express possibility in affirmative sentences.
It may rain tomorrow.
He may be at home.
May is also used to express a wish.
May you live long!
May success attend you!
Might is the past tense of may and is used to express a degree of dissatisfaction or reapproach.
He cried aloud so that his friends might hear him.
You might see me tomorrow.
You might pay a little more attention to your appearance.
Might is also used in polite request.
Might I have your umbrella for a day?
Shall is used in the first person to express pure future.
I shall do this work.
When shall we visit the zoo?
Shall I do it for you?
Tomorrow we shall meet our uncle.
Shall is used to express command, desire, promise or threat etc., in second and third person.
Shall you go tomorrow? (desire).
He shall not enter my house (command).
You shall have a surprise tomorrow (promise).
You shall be punished for unfair means in examination (threat).
Will is used in the second and third persons to express pure future.
Tomorrow will be Sunday.
You will see that I am correct.
Will is used to express volition.
I will (= am willing) to carry your luggage.
I will (= promise to) try to do better the next time.
Will is used to express characteristic habit.
He will talk about nothing but politics.
She will sit for hours watching the television.
Will is used to express assumption or probability.
Must is used to express :
(i) Necessity or Obligation.
We must obey our parents.
One must do his duty.
(ii) Fixed determination.
I must have my way in this matter.
He must be fifty now.
Ought is followed by an infinitive and it expresses :
(i) Moral obligation, duty or desirability.
You ought to have come in time.
We ought to love our parents.
Need not, Dare not
Need is commonly used in negatives, which denote necessity or obligation.
He need not go there. (It was not necessary for him to go.)
I need not have bought it. (It was not necessary for me to buy it, but I bought it.)
Dare is generally used in negative sentences, meaning be brave enough to.
He dare not take such a step?
He dared not to do it.
Do is used :
(1) To form the negative and interrogative of the present simple and past simple tenses of non-anomalous verbs.
He doesn't talk.
He didn't do.
Does she talk?
Did she do?
(2) To avoid repetition of a previous ordinary verb.
Do you know her? Yes I do.
She sings well. Yes, she does.
You called him, didn't you?
He eats apples and so do you.
(3) Do is also used to emphasize the affirmative nature of a statement.
As, You do go there.
I told him not to do, but he did do.
(4) Used is followed by the infinitive to. Used to is used to express a discontinued habit.
As, I used to live there during 1980s.
There used to be a house in the garden. Used to is an anomalous verb.