The nine basic modal auxiliaries helping verbs are can, could, will, would, shall, should, may, might, and must. There are a few others as well, in particular ought to, which is close in meaning to should. Occasionally need can be a modal rather than a main verb.

The nine basic modals fall into the pairs can/could, will/would, shall/should, may/might, and the loner must. In earlier English, the second member of each pair was the past tense of the first. The second form still functions occasionally as a past tense, especially in the case of could.

Ingrid can ski.

Ingrid could ski when she was five.

But, for the most part, in present-day English, all nine modals typically refer to present or future time. When you want to use a modal to refer to the past, you follow the modal with a perfect form of a helping verb.

If you have a fever, you should see a doctor.

If you had a fever, you should have seen a doctor.

In the case of must, refer to the past by using had to.

You must renew your visa by the end of this week.

You had to renew your visa by the end of last week.

Using modals to make requests or to give instructions

Modals are often used in requests and instructions. Imagine making the following request of a flight attendant:

Will you bring me a pillow?

This request may appear demanding or rude. Using a modal makes the request more polite by acknowledging that fulfilling the request may not be possible.

Can you bring me a pillow?

Another way of softening the request is to use the past form of will, and the most discreet choice is the past form of can.

Would you bring me a pillow?

Could you bring me a pillow?

Using the past tense of modals seems more polite than using their present forms because it makes any statement or question less assertive.

Consider the meanings of each of the following instructions:

Instructions 1 and 2 give permission to submit the report electronically but do not require it; 2 is more formal. Instruction 3 makes a strong recommendation; 4 allows no alternative; and 5 implies, “Don’t even think of doing otherwise.”

Using modals to indicate doubt or certainty

Modals can also indicate how confident the writer is about his or her claims. Look at the following set of examples, which starts with a tentative suggestion and ends with an indication of complete confidence:

The study might help explain the findings of previous research.

The study may help explain the findings of previous research.

The study will help explain the findings of previous research.