cant the coordinating conjunction act as subordinating in any case?
Coordinating conjunctions are coordinators that can join two words, clauses or sometimes even sentences.
They can't play nor can they swim;
I reached early although I was not interested.
She lied yet she was arguing.
Subordinating conjunctions differ from coordinating conjunctions because they can only join clauses and not words. They can conjoin independent as well as dependant clauses.
He waited to open the envelope till it was dark at night;
I wonder whether my mother is asleep by now;
They knew that she would be on time.