There’s no doubt that “Be happy” and “Don’t be sad.” are correct, and “They be happy” is incorrect.
But is it correct to say:
Why don’t you be more careful?
“Why don’t they be happy?” ?
Why is each of these wrong or right. Is it acceptable to use be and do together in this context?
I think it is difficult to find a counterexample, but “be” can be used in sentences in subjunctive mood:
- I suggest/advise/recommend that you be quiet
- It is necessary/essential/imperative/vital that he be here on time.
However, in these examples “be” is not the main verb. The following are examples of phrases where be is the only verb; they are, however, idiomatic expressions (in subjunctive mood)
- if need be
- far be it from me
- be that as it may
More examples with “be” being the only verb can be found:
- Be you angels? And we said… Nay, we are but men! (Tenacious D.)
- To be or not to be?