She kept weeping while watching the movie.
She wept while she was watching the movie.
“while” is not a preposition at all in modern english. It is a subordinating cinjuction, which a clause resulting in the formation of a subordinate clause.
while wtching the movie____is the elliptical version of the clause “while she was watching the movie” because we do not generally repeate the subject when it functions two actions.
Many people campare “while watching” to “before going” which is a prepositional phrase , but “while watching” is not, it is an elliptical adverb clause, which must be written in its full form.
we can write “while in the market”, but we cannot write “before in the market” as a oreposition is not followed by a another prepositional phrase, which is functioning as a predicative expression.
Just as we dont say___ He went there and killed his enemy, we dont also say “ I was watching TV while I was sitting there”.
we must write these sentences__
He went there there and [ he ] killed his enemies.
I was watching TV while [ I was ] sitting there.
Whenever you find [while+Ving], just expand it to a full adverb clause.
Note___After some subordinators as; whether, though, while, if, we can leave the subject and the predicative verb “be”
have a look at this sentences___
If possible, please help me.
If [ it is ] possible.
though poor, he is happy.
though [ he is ] poor.
Do not come until sent for.
Until [ you are ] sent for.
when done, let me know.
when [it is] done.
while driving, be careful.
while[ you are ] driving.
All these sentences are complex sentence with elliptical adverb clauses.
We can have a simple sentence with a subordinator.
I hope it helps🙏