You're not going to understand this if you think of it as only
the two words "be it". It's a construction, and it has quite a lot
of other stuff going on.
First, it occurs only in a subordinate clause of a particular
kind: there are always at least two (usually reduced) clauses,
disjoined with an "or"; the implication is that either clause is
possible, but it's not known which one.
So it alternates with "whether" clauses.
(BTW, "whether" is just the wh-word used with Yes/No
questions when forming an embedded question clause:
I don't know the answer to the question "Where is he?".
===> I don't know where he is.
I don't know the answer to the question "Is he here?".
===> I don't know whether he is here.)
1) Be it the rain or the company, it doesn't look like fun.
2) Whether it's the rain or the company, it doesn't look like fun.
You can only use "be it" initially if the verb in the "whether"
clause is "be". If you use a different verb, you can't front it:
3) Whether he goes north or south, it'll take him 3 hours.
4) *Go he north or south, it'll take him 3 hours.
If it's a form of "be", however, it can be fronted, but only as "be",
never as "is", "am", or "are". This is an infinitive form, which
substituted for the subjunctive forms after they were lost in
English.
(If you want to see what subjunctive verb forms really are like,
take a look at the paradigm of the German verb "sein" 'to be'.)
5) Whether he is alive or (he is) dead, I'll crush his bones to make
my bread.
6) Whether he be alive or (he be) dead, I'll ...
7) Be he alive or (be he) dead, I'll ...
The "be it" form always corresponds to a whether-clause
with a structure: Whether it is... or (it is) ..., S. These are
pretty frequent, especially in formal language. "It is" can
be manufactured ad lib by using a cleft construction, which
produces a dummy "it" and a dummy form of "be":
8) He likes the fried potatoes.
===> (Cleft)
9) It is the fried potatoes that he likes.
The "be it" construction is called "subjunctive" by some
people because
a) it uses an odd verb form
b) it uses an odd word order
c) it expresses some possibility and lack of knowledge
This is not what "subjunctive" means, but it is what is often
taught to students in Anglophone schools as "subjunctive".
-John Lawler
http://www.umich.edu/~jlawler/aue/complmnt.html
Happy Hogswatch All and May Gods Bless Us, Every One.
(Atheists may request the vegetarian alternative.)