Can – Could – (be) able to


To show ability

To suggest a possibility or to give an option

To ask for or to give permission

To show impossibility


I can run 10 miles.

I could run 10 miles when I was in high school.

May – Might


To ask for or to give permission (formal)

To show possibility


The instructor may come to class late today.

The instructor might have come to class late yesterday.

Must – have/has/had to


To show necessity

To show lack of necessity


I am glad that I don’t have to cook tonight.

We had better leave. It is getting late.

Will – Would – (be) going to


To indicate future time

To make a promise or to show willingness

To state a general truth

To ask a polite question


The federal government will provide assistance to the hurricane victims.

Mark promised that he would help me with my math homework.

Shall – Should – Ought to


To show advisability

To show advisability after the fact

To show obligation

To show an obligation that was not carried out

To show expectation

To show an expectation that was not realized


I should renew my driver’s license. It expires next month

You ought to receive my letter in two days.


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