Is it correct to use the following structure in the English language if plus subjet plus infinitive eg If he be or if she do instead of is and does?

Answer 1

It is correct to use this subjunctive form--the verb is not infinitive but subjunctive-- in the if clause if the principal clause does not state or imply what would be or would have been and if the subordinate conditional clause indicates an action that is contemplated or planned but also connotes some degree of reserve on the part of the speaker. Here are four examples:

If this be true, we are all wrong.

Should this be true, we shall all be wrong.

If it be inevitable, why do you complain?

Whether it be safe or unsafe, the bridge will have to be crossed.

Source: Usage and Abusage, by Eric Partridge, revised by Janet Whitcut

Answer 2

The structure "if he be" or "if she do" is not commonly used in modern English. Instead, it is more appropriate to use "if he is" and "if she does" for present tense verb forms. This follows the standard subject + verb structure in English grammar.