Today's English lesson

From the American Heritage Book of English Usage:

You Must Needs Use
Auxiliary Verbs, But You
Ought Not Use Them Improperly

The auxiliary verb must is used to express necessity, obligation, and probability:

Plants must have water in order to live.

Swimmers must take a shower before entering the pool.

You must be joking.

Unlike other auxiliaries like can and may, must has no past form like could or might:

They insisted we must wait until tomorrow.

In this regard must resembles need and ought to.

Depending on the sense, the verb need behaves sometimes like an auxiliary verb (such as can or may) and sometimes like a main verb (such as want or try). When used as a main verb, need agrees with its subject, takes to before the verb following it, and combines with do in questions, negations, and certain other constructions:

He needs to go.

Does he need to go so soon?

He doesn’t need to go.

When used as an auxiliary verb, need does not agree with its subject, does not take to before the verb following it, and does not combine with do:

He needn’t go.

Need he go so soon?

Unlike can and may, however, auxiliary need has no other form like could and might for the past tense:

He said we need not worry about that.

ought as auxiliary verb. Ought is an auxiliary verb that usually takes to with its accompanying verb:

We ought to go.

Sometimes the accompanying verb is dropped if the meaning is clear:

Should we begin soon? Yes, we ought to.

In questions and negative sentences, especially those with contractions, to is also sometimes omitted:

We ought not be afraid of the risks involved.

Oughtn’t we be going soon?

This omission of to, however, is not common in written English. Like must and auxiliary need, ought to does not change to show past tense:

He said we ought to get moving along.

ought in regional expressions. Usages such as

He hadn’t ought to come.

and

She shouldn’t ought to say that.

are common in many varieties of American English. They should be avoided in written English, however, in favor of the more standard variant ought not to.