Modal verbs, also known as the helping verbs, are the most important parts of speech that are used to express ability, possibility, permission and obligation. A lot of times, we get confused or make errors in using can/could, will/would, may/might, shall/should. Given below is how the following should be used.

Can and Could

Can is used in sentences to express ability when using in affirmative sense and the past of can is could.

I can climb.

I could climb. (I had the ability to climb may be when I was young, and now I am not able to!)

Can and could both are used in interrogative for permission. Could is considered a more formal and polite form.

Can I borrow your pen?

Could I borrow your pen?

Will and Shall

Will is used used for showing intention, desire of some action in future.

I will become a doctor when I become old.

Shall is used to ask for suggestion

Shall we go and dance?

Sometimes shall is used to indicate obligation in formal communication (notices, announcements, applications etc.)

The employees shall get to their departments by 9 a.m.

Will and Would

Will expresses certainity of something that has to happen in future whereas would is less certain and has a conditional mood.

The bus will reach by 9 a.m.

She would finish the book. (There is an implied sense or a condition like if she were upto.)

Would is also used for some action that continued to happen in past, instead of ‘used to’.

When she was a kid she thought that she would become a doctor.

Would is the past of will when indicating future in the past.

In summers we would go to a new beach every year.

Would is also used to express wish or preference.

I would like to have a cup of coffee.

I would rather go for a movie than sit at home.

Would is also used for polite offers and invitations.

Would you mind passing the pen please?

Hypothetical sentences- Such sentences expresse factual implications or hypothetical situations (also known as the if clause) and their consequences (known as main clause). These express situations which are imaginary or unrealistic. They can be of the following forms

If I were you, I would never go out for work. (The if clause takes the second form of verb after subject and the main clause takes would plus first form of verb with the subject.

If I won a billion dollars, I would travel to the moon.

Note: We never use will and would in the clause containing conditional words like if, when, after, wherever, whenever etc.

 If you will come, I can go is wrong.

If you come, I can go is right

In indirect speech, will becomes would.

She says, “ I will win the match”.

She said that she would win the match.

Shall and Should

Shall indicates obligation in very formal situations.

The candidates shall not bring their phones in the premises.

Should is used to give opinion or suggestion and to ask for opinion and suggestion.

You should call for a taxi, if you want to reach early.

Should we tell him about the movie?

What should I do?

In indirect speeches, shall becomes should.

He says, “ I shall get a taxi”

He said he should get a taxi.

May and Might

May and might indicate probability in future. While may is more certain, might carries with it lesser probability and applies to hypothetical and counterfactual situations.

She may come is more sure than She might come.

Might is the past of may.

She thinks she may become a doctor.

She thought she might become a doctor.