[Is no subject here ok?] Please turn off the light(s) before leaving the room.
[Is no subject here ok?]Please turn off the light(s) before leaving the room.

I saw a [sentence a].
a .[Please turn off the light(s) before "you" leave the room.]

How about the following [sentence b]? Is it grammatically correct, or not?
b. [Please turn off the light(s) before leaveing the room.]
--
My openion
1. And I know the grammar thet the subject can be omitted if the subject is the same in both the main clause and the subordinary clause{adverb}.

2. But when i searched online for [without subject ... (imperitive + adverb clase)], I didn't find any evidence or anyone talk about it.

My question is whether the [sentence b] is grammatically correct, or not. Is it work?
-
Any tips or suggestions are all welcome.
If there is anything wrong in my opinion or understanding, please help to correct it.
Thank you for your attention.

Question

[Is no subject here ok?] Please turn off the light(s) before leaving the room.
[Is no subject here ok?]Please turn off the light(s) before leaving the room.

I saw a [sentence a].
a .[Please turn off the light(s) before "you" leave the room.]

How about the following [sentence b]?   Is it grammatically correct, or not?
b.  [Please turn off the light(s) before leaveing the room.]
--
My openion 
1. And I know the grammar thet the subject can be omitted if the subject is the same in both the main clause and the subordinary clause{adverb}.

2. But when i searched online for [without subject ... (imperitive + adverb clase)], I didn't find any evidence or anyone talk about it.

My question is whether the [sentence b] is grammatically correct, or not. Is it work?
-
Any tips or suggestions are all welcome.
 If there is anything wrong in my opinion or understanding, please help to correct it. 
 Thank you for your attention.

Answer

a . Please turn off the light(s) before you leave the room.

b.  Please turn off the light(s) before leaving the room.

Yes, both sentences are correct. They are just Imperatives - where the subject is not stated - it is understood.
It's like: "Come here!" or "Sit down, please." - It is not necessary to say "You come here" or "You sit down, please", because you are addressing the person directly. You can onit the subject in this case.
However, you might use the subject in certain situations - for example, if you were addressing a group of people, saying "Come here." and the wrong person approached, then you might indicate another person, saying "(No) you come here." i.e. not the first (incorrect) person.
Hope that's clear...

Answer

Thank you, Michael, for your nice explanation. 
Thank you,, Phil, for your correction and the good intention to help solve the follow-up questions.

Answer

They’re both correct. We normally don’t state the subject in the imperative. As far as “before leaving the room”, this is not a clause — it’s a gerund phrase, and so functions as a noun. It’s the same structure as “wash your hands before dinner”. If you have any follow-up questions, inbox me, as there are no notifications otherwise.

Answer

English sentence structure is surprisingly flexible, and many forms beyond the obvious and basic are possible and even, sometimes, "correct" - grammatically OK.

Your example sentence is called an imperative, a command. The subject is implied; it is the reader or listener.  This is from a grammar site:

"Normally when we use the imperative there is no subject because the subject is obvious—it's YOU! Sometimes, however, to make the subject clear, we do use a subject, for example: Everybody look! Relax, everybody."