be with auxiliary do

Normally, be is used without the auxiliary do.

• I’m not often sick. (not I don’t often be sick.)

But do is used to make negative imperative sentences with be (when we tell somebody not to do something).

• Don’t be silly!
• Don’t be such a nuisance!

And do be can begin emphatic imperatives.

• Do be careful!
• Do be quiet, for God’s sake!

In an informal style, people sometimes use do with be in one or two other structures which have a similar meaning to imperative sentences.

• Why don’t you be a good boy and sit down?
• If you don’t be quiet you’ll go straight to bed.

note

For other auxiliary uses of do, (see here).