I am wondering if my sentences are grammatically correct.
When after their holiday, Gina told her husband, Phil, she wanted a divorce, he was sure she had planned that months in advance. He felt stupid and taken advantage of. He remembered his mother's warning that he was naive and inexperienced and easy prey for cunning women.
I would make several changes to sentences.
First of all, I find the beginning very awkward. I think you are after a subordinate clause with the use of "when" and that makes sense.
"
When [STRIKE]
after their holiday,[/STRIKE]
Gina told her husband, Phil, she wanted a divorce..."
When "after the holiday" is removed then suddenly the clause is easy to see and read. I would place that prepositional phrase at the start of the sentence, out of the clause.
"After their holiday, when Gina told her husband, Phil, she wanted a divorce,...."
This sounds a lot better, right? Now here is the complete sentence.
"
After their holiday, when Gina told her husband, Phil, she wanted a divorce, he was sure she had planned [STRIKE]
that[/STRIKE]
this months in advance."
I think the second sentence could be written better. Think about it! Doesn't it seem
choppy to you, nestled between two great sentences?
The last sentence is ok but I would remove the first "and" and insert a comma.
He remembered his mother's warning that he was naive, [STRIKE]
and[/STRIKE]
inexperienced and easy prey for cunning women.
Think about rewriting that second sentence. It is not wrong, but I challenge you to make it better.