Modal verbs are helping verbs that go before regular verbs and add a new shade of meaning to the sentence – a degree of ability, possibility, or permission. The easiest way to learn modals is in order of their strength and degree.

(STRONGEST)
MUST
WILL
HAVE TO
OUGHT TO
WOULD
SHOULD
CAN
COULD
MAY
MIGHT
(WEAKEST)

These modal auxiliary verbs are easy to use in that they are invariable even in the third person singular. They do not change in number based on the subject. They are used to express ABILITY, POSSIBILITY, REQUESTS, PERMISSION, OBLIGATION, and LOGICAL CONCLUSION.

The best thing is probably if I write a sentence to show you how they are used.
Students get the most confused between “should, would, and could.”

You must get that report finished by tonight!
I will get it done before the deadline.
You have to help me, or I will never finish it in time.
You ought to start earlier so that you do not end up in such a pressured situation.
I would offer to help you, but I have too much work of my own to do.
I should call my professor and find out if the deadline can be extended.
You can call, but I don’t think your deadline will change.
I could always pretend to be sick.
He may ask for a doctor’s note.
I might just have to work all night.

We also use modal verbs to make POLITE REQUESTS and to ask for ADVICE.

Could you please answer the phone?
Would you give me a ride to the station?
Would you mind giving me a hand with this task?
May I ask your name?
May I take this seat?

Should I apply for the position?
What would you do if you were in my shoes?
Could you please advise me as to what to do next?
What would you suggest that I do next?
What would you recommend that I say during the interview?

The modal verbs “can” changes to “could” when we talk about a past event.
We use “could” or “was able to” to talk about ability in the past.
We use these expressions to say someone successfully did something.
Present simple – I can swim.
Past simple – I couldn’t swim when I was younger. I wasn’t able to overcome my fear.

The modal verb “will” is replaced with “would” to talk about certain past actions.
Future: She will close the window.
Past simple: He said he would close the window, but he forgot to do it.

Did you know that although modal verbs are only used in the present simple, they can be paired with the verb “to be“ or the verb “to have” to take on different meanings in different tenses?

To use a modal in the continuous tense, we add “be” plus the present participle.
He must be going abroad because he has his passport with him.
He will be going abroad right after graduation.
He could be going abroad as soon as next week.
He would be going abroad if his plans had not changed.
He should be going abroad in my opinion, but he says he doesn’t want to.

Here are some examples of modals in other tenses:
Past simple – I had to finish my test.
Future – You will have to give me your phone number.
Future – I’m not sure if I will be able to leave the country.
Present perfect – I haven’t been able to leave the house.

“Would have,” “could have,” and “should have” are sometimes referred to as modals of “lost opportunity.”

    I thought I saw my cousin in the street. It couldn’t have been him because he is in Miami.
    I would have liked to take the faster train, but I didn’t know which one it was.
    I should have agreed to marry you when I had the chance.

We use a modal followed by the present perfect (also called the perfect infinitive) formed by “have” + the past participle

– to refer to the past:
    I may have been wrong about you.

– to refer to unreal situations:
    I could have won the competition, if that professional athlete did not compete against me.

– to show that an activity was different than what we wanted:
    I would have liked chocolate, but she served me vanilla.

– to say how confident we are that something has happened (to make deductions and guesses/speculations):
    It must have been the dog who knocked over the lamp.
    It might have been my little sister who knocked it over.
    It may have even been the cat.
    She speaks English so well. She might have spent time abroad.
    Yes, I agree. She must have spent time abroad.