They always come first in a
verb formation, before other
auxiliaries and the main verb:

{modal} + [auxiliary] +
{main verb}

Modal Verbs are special verbs
which behave very differently
from normal verbs.

Here are some important differences:

       1. Modal Verbs do not
       take "-s" in the third person.

       The verb which follows is
       always in the infinitive form:

Examples:
He can speak Chinese.
She should be here by 9:00.
He might be...
They should stay...
She could have...

       2. You use "not" to make Modal        Verbs negative, even in
       Simple Present and Simple Past.

Examples:
He should not be late.
They might not come to the party.

3. Many Modal Verbs cannot be used in the past tenses or the future tenses.

Examples:
He will can go with us.
NOT CORRECT

She musted study very hard.
NOT CORRECT

1. the word TO cannot go
before or after these verbs:
example:
I can't swim ----
Remember: I can't to swim is incorrect.

2. There is no different form
for the 3rd person singular present.
example:
He can swim ----
Remember: He cans swim is incorrect.

3. They do not use AUXILIARY VERBS
in negative or interrogative
example:
Can you play chess? ----
Remember: Do you can play chess? is incorrect.

4. None of these verbs, except
CAN, (COULD) has a past tense form.
To put the rest in the past tense
we must add HAVE + PARTICIPLE.

We can use them with
the -ing form to show
continuous modality:

{modal} + {be} + {-ing}

He might be working late.
The should be coming soon.
He must be having a party.

We can use them with the
past participle to show past modality

{modal} + {have} + {past participle}

He might have been working late.
The should have seen him.
He must have gone away.

We can use be able to, be allowed to
and have [got] to after modals:

{modal} + {phrase}

I will be able to...
She might not be allowed to...
They should have to...

Note: we do not use these in
continuous forms.

We use modal verbs to show:

what we think or feel about a situation:

They should not have gone.
They might have been working hard.

our ability to do something:

necessity:

possibility:

To show degrees of ability,
we can use modal verbs. These show:

ability or opportunity in the present:

She can play the piano.
She is able to play the piano.

ability or opportunity in the past:

She could play the piano.
She was able to play the piano.

ability in the future:

He could do your job given the opportunity.
I will never be able to lift that cupboard alone.

to show ability; we often use can or
be able to with verbs of perception:

I can hear him but I cannot see him.
I am able to hear him but I am not able to see him.

can v. be able to:

In most situations, be able to is
more formal than can.

I could leave.
I was able to leave.

Both examples say I had the
opportunity or ability to leave,
but could means I did not leave
and was/were able to means I did leave.

When we talk about a real past action
we use was/were able to and
when we talk about a hypothetical
course of action we us could.

When we talk about a skill, could and
was/were able to mean the same thing:

I could swim when I was four.
I was able to swim when I was four.

Note that in conditional sentences also, could and be able to are interchangeable.

Can I open the window?
Could I open the window?
Am I able to open the window?

In questions, can and could
are requests, while be able to
asks about ability.

100% necessity:

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You must leave straight away.
You have [got] to leave straight away.

must v. have [got] to

Generally, must shows what the speaker feels and it is the
speaker's authority;

have [got] to is when the speaker uses an external force which
they cannot control:

We don't have to pay to get in.
We have not got to see the boss after all.
We needn't stay late.

You ought to call your mother.
You should call your mother.

These show that calling your mother is
the right thing to do. There is no real difference between
ought to and should, but ought to
is perhaps a little stronger.

We use needn't to show an action is optional -
I can do it if I want to.
We use mustn't to say an action is forbidden -
I have no choice.

To show degrees of offers,
we can use modal verbs:

I will give you hand with that case.
Shall I give you a hand with that case?
Can I give you a hand with that case?

We use shall as let's but the
person has a chance to refuse:

Shall we go to the cinema?

To show degrees of permission,
we can use modal verbs:

permission

You cannot leave yet.
You are not allowed to leave yet.
You may not leave yet.
You must not leave yet.

can is more friendly than may
which is quite formal.

needn't or mustn't

We use needn't to show an action is optional -
I can do it if I want to.
We use mustn't to say an action is forbidden -
I have no choice.

giving orders:

You will not leave yet.

To show degrees of possibility,
we can use modal verbs:

imagining things:

That would be nice.

can or may

It can get very tense in some of these meetings.
It may get very tense in some of these meetings.

To talk generally about something we use can,
but talking about a specific set of something,
we use may. In the first example the speaker makes
a general comment about the meetings;
in the second example the speaker is talking about
a specific group of future meetings only.

We could go to the cinema.

could is more polite thancan.