Consider the following phrases (someone is talking with a friend who is challenging some mobsters):
If you are not careful, they will catch you.
If you don't be careful, they will catch you.
Is the second option acceptable or usual? Are they both in according with grammar rules?
SHORT VERSION: As to whether or not both versions are acceptable, the answer is:
- Yes, both versions are grammatical and are considered to be standard English.
In your 1st version, the verb "are" is an auxiliary BE being used in a typical copular usage (usage similar to "If you aren't careful …"). In your 2nd version, the verb "be" is behaving as a lexical be--but there are constraints on where the lexical be usage can be used.
This explanation is based on my interpretation of the info that is in the 2002 CGEL.
Your version #1 ("are not") might be considered to be slightly more formal of a style than a version that uses "aren't":
1.a If you are not careful, they will catch you. -- (original #1)
1.b If you aren't careful, they will catch you.
Both #1a and #1b are using the auxiliary verb "BE" where the verb "BE" is being used as a copular verb. And both versions are fine.
As to your version #2 ("don't be"), the verb "BE" is behaving as a lexical verb, taking do-support in a present-tense negative construction:
- If you don't be careful, they will catch you. -- (original #2)
Some background info: CGEL considers there to be six main uses of the verb lexeme BE. In CGEL page 113:
We distinguish the following uses of be:
- i. She was a lawyer. -- [ copula be]
- ii. She was sleeping peacefully. -- [ progressive be]
- iii They were seen by the security guard. -- [ passive be]
- iv. You are not to tell anyone. -- [ quasi-modal be]
- v. She has been to Paris twice already. -- [ motional be]
- vi. Why don't you be more tolerant? -- [ lexical be]
Your version #2 is similar to [62.vi ], which involves a lexical be usage.
In CGEL page 114 has info related to lexical be usage, including examples:
This is found with why + do and with if:
- i.a. Why don't you be more tolerant?
i.b. Why doesn't he be more tolerant?
ii.a. If you don't be quick you'll lose.
ii.b. If he doesn't be quick he'll lose.
iii.a. % If you be quick you'll win. -- (grammatical in some dialects only)
- iii.b. * If he be / bes quick he'll win. -- (ungrammatical)
Their discussion of  is:
The why construction [i ] is virtually restricted to the negative: ? Why do you be so intolerant? is at best very marginal. Pragmatically [i ] conveys "You/He should be more tolerant" and thus bears some resemblance to the imperative, but syntactically it is quite distinct from the imperative construction by virtue of having a present tense form, not a plain form. This is evident from the person-number contrast between don't in [i.a] and doesn't in [i.b], for imperatives with a 3rd person singular subject do not differ in verb-form from those with a 2nd person subject (cf. Somebody open the door, please).
The same person-number contrast is seen in the conditional construction [ii / iii ], which again conveys that you/he should be quick (in order to win / avoid losing). This time, however, some speakers allow be in the positive, but with no corresponding 3rd person singular form.
With additional info:
Two points about be follow from the data of . The first is that in these constructions it behaves as a lexical verb, taking do-support in present tense negatives. The second is that for speakers who use construction [iii.a] the lexical and auxiliary uses correspond to different lexemes, for the inflectional forms are different. Lexical be has only the one realisational form be, but it realizes either the plain form (when taking do-support) or (in positive conditionals) a present tense form, distinct from the are that we have with auxiliary be.
What the above CGEL info means is that, with respect to the lexical be usage, there are two dialects of standard English: Dialect A, Dialect B.
DIALECT A: The speakers of this dialect consider only the unmarked examples in , which are the four examples in [63.i-ii ], to be grammatical:
The examples are all negatives and used under the constraint of being in constructions involving "why + do" or" if". The examples are using the auxiliary verb lexeme "BE".
These speakers do not accept [iii.a], which is ungrammatical to them: * If you be quick you'll win.
DIALECT B: The speakers of this dialect accept the four examples in [63.i-ii ] that the Dialect A speakers accept, and also accept example [iii.a ] as grammatical, which is now unmarked:
- iii.a. If you be quick you'll win.
The reason why this example is not acceptable for Dialect A speakers is because it is not a negative.
Dialect B speakers consider all the lexical be usages to be using a different verb lexeme, which is a lexical "BE" verb lexeme, and it is different from the auxiliary "BE" lexeme. That is, they have two different verb lexemes, while Dialect A speakers have only the one standard auxiliary "BE" lexeme.
The reason why Dialect B speakers consider that the lexical be usage to be using a different verb lexeme -- the lexical "BE" verb lexeme -- is because it has its own present tense verb form: "be". For instance, the lexical "BE" lexeme's 2nd person form is "be", while the auxiliary "BE" lexeme's 2nd person is "are".
This lexical "BE" lexeme is rather defective in that it only has a plain form and present tense form, and even the present tense form is defective in that it has no 3rd person singular form.
NON-STANDARD DIALECT: As to example [63.iii.b], it is ungrammatical for standard English. This is why it is marked with "*":
- iii.b * If he be / bes quick you'll win.
Notice that the example ("he be/bes") uses 3rd person singular, which is not acceptable for either Dialect A or Dialect B speakers.
ASIDE: Note that for many speakers, there is a non-standard alternate for [63.ii.b] in informal or colloquial style:
63.ii.b. If he doesn't be quick he'll lose. -- (standard, CGEL)
alternate: ! If he don't be quick he'll lose. -- (non-standard)
And also, as to [63.iii.b], there is a non-standard alternate for some speakers:
63.iii.b. * If he be / bes quick he'll win. -- (ungrammatical, CGEL)
alternate: ! If he be quick he'll win. -- (non-standard)
Note that CGEL is the 2002 reference grammar by Huddleston and Pullum et al., The Cambridge Grammar of the English Language.