Modal Verbs

Modal verbs provide additional and specific meaning to the main verb of the sentence.

The modal verbs in English are
"can," "could," "may," "might," "must," "ought," "shall," "should," "will," and "would.“

They are special verbs which behave irregularly in English.
They never change their form. You can't add "s", "ed", "ing"...

They are always followed by the main verb. E.g. can wash, may go, must clean, and ought
to go
How to use a modal verb in a sentence

Active voice
Affirmative (Positive statement) - You can do the homework later.
Negative (Negative statement) – You cannot do the homework later.
Interrogative (Question) – Can I do the homework later?

Passive Voice
Affirmative (Positive statement) – The homework can be done later.
Negative (Negative statement) – The homework cannot be done later.
Interrogative (Question) – Can the homework be done later?
Use of Modal verbs

Modals have a great variety of communicative functions such as showing obligation,
permission, prohibition, ability, advice, recommendation, possibility and probability.

To show obligations
We use must to talk about obligations:
E.g.
Everyone must wear lab coats during lab sessions.
E.g.
Dangerous chemicals must be handled carefully.
(Note: We also use have to when talking about obligations. It has the similar
meaning to must but changes its form like a non-modal verb’
E.g.
Do I have to wear a lab coat during lab sessions?)
There is no past form of must. We use ‘had to’ to talk about past obligations.
E.g.

When I was a child I had to walk to school everyday.
To show permission and prohibition
We use may, could or can to talk about permission.
E.g.
May I borrow your textbook?
We use cannot when talking about not having permission to do something.
E.g.
This is not an open book exam. You cannot refer your notes and books
during the exam.
We use must not to talk about prohibitions.
E.g.
You must not perform unauthorized experiments in the lab.
(meaning -Do not do it!)

To show ability
We use can and cannot to talk about someone's skill or general abilities:
E.g.
Green plants can produce oxygen by photosynthesis.
Dogs cannot see colours as well as humans do.
We use could and couldn't to talk about the past:
E.g.

When I was young, I could run fast.
To show advice and recommendation
We use should/should not to give advice.
E.g.
You should study hard for your exam.
The Work top should be cleaned properly before leaving the lab.

To show Possibility and probability
This use of modals hedges, or weakens, the certainty of a sentence. The stronger the
modal, the stronger the possibility.
must -
to say that we believe very strongly that something is true.
E.g. Those clouds must mean that it will rain later.
Can
- to make general statements about what is possible. It suggests a strong
possibility.
E.g. Vitamin C can boost your immune system.
may - to say that something is possible, but not certain. In scientific usage, may is
considered to be a little stronger than might and could.
E.g. The bottle may break when dropped. (Meaning- Some chance that it will
break.)
might /could - The weakest modals that can be used to show possibility.
E.g. The bottle could break when dropped. (Meaning- A small chance that it will
break.)
Cannot /Could not - to say that we think that something is impossible.
E.g.

New technology cannot bring happiness to mankind.
To show past deductions
must have + past participle form of the main verb -when we feel sure about what
happened.
E.g.
The students must have done their homework. - (Active voice)
Their homework must have been done by the students. - (Passive voice)
might have / may have / could have + past participle form of the main verb -when we
think it's possible that something happened.
E.g.
The students may have done their homework. – (Active voice)
Their homework may have been done by the students - (Passive voice)
can't have / couldn't have + past participle form of the verb- when we think it's not
possible that something happened.
E.g.
They can’t have gone out in the rainy weather.