Modal Auxiliaries

Modal verbs are a kind of auxiliary verb. They facilitate the main verb for suggesting potential, expectation, permission, ability, possibility, and obligation.

When used with the main verb, modal verbs do not end with -s for the third-person singular. Modal auxiliary verbs never change form, but they have a different form for past tense.

The modal auxiliaries include:

Present Tense	Past Tense
Will Can Must (have to) May Should (ought to) (had better)	Would (used to) Could (Had to) Might Should (ought to)

NB: The words in parentheses ( ) are semi-modals. They have the same meaning, but they are different grammatically.

Will – Would

Will indicates a ‘willingness’ to do something in the future. The negative form of will – will not (won’t)indicates an ‘unwillingness’ (refusal, reluctance) to do something.

Example:

I will give you another opportunity.
I will play tomorrow.
They will arrive at 10 AM.
She won’t come today.

Would indicates general or repeated willingness in the past. It also indicates preference in the present.

Example:

If you did not leave, I would still be taking care of you.
Whenever I had to go there, they would throw a party.
We thought that people would buy this book.
If I were you, I would not do it.
I would like to make a toast.

Used to sometimes replaces would but sometimes it would be grammatically incorrect if we use used to in place of would.

Example:

When I was in school, I used to make sketches.
He often used to cry at night without reason.
I used to take a break at this time of the year.

Can – Could – May – Might

These modals express possibility and ability.

Can indicates ability. Could indicates ability with an option.

Example:

I can do it. (The subject ‘I’ is sure about his/her ability)
I could do it. (The subject ‘I’ is not sure about his/her ability)
They cannot do it. (present)
They could not do it. (past)

Can & could also indicate possibility.

Example:

The temperature can rise this month.
They can’t go too far by now.
It could rain later.

May and might both indicate possibility but might can suggest that there is less possibility than may.

Example:

It may rain later.
It might rain later.
They may come back.
They might come back.

Must

Must indicates necessity.

Example:

I must leave now.
He must study hard.
Alex must go home by 6.00 pm.

Have to has a similar meaning to must but implies less urgency.

Example:

I have to leave now.
He has to study hard.
Alex has to go by 6.00 pm.
I had to leave then. (past)
He had to study hard to pass the exam. (past)

Should

Should indicates obligation and probability.

Example:

You should come home early.
You should not smoke at all.
I should visit my parents more often.
There should be an extra key for the lock in the drawer. (probability)
He should have reached by now. (probability)
I should have done that. (obligation in the past)

Ought to and had better sometimes replaces should.

Example:

You ought to come home early.
We ought to have taken a taxi. (Past)
We had better leave. (Had better is generally used in spoken English.)
I think parents ought to give children more freedom. (Had better won’t be appropriate here.

Source: Learn Grammar