Coordination draws an equal emphasis on the two clauses of the sentence. Usually a coordinating conjunction is used to join two clauses in which events of both the clause are of equal importance.

He was driving the car and a smoking cigar.
In the above mentioned sentence, the subject is doing two activities, both of which are of equal importance.

Subordination plays down one of the clause by showing that one clause is cause/responsible/reason for the other clause.

While he was driving the car he was also a smoking cigar.
Here the subordinating conj "also" downplays the second clause and hence acts as a subordinator.

Do not do Subordination where Coordination is required(vice-versa)
Example :
XXX The Economy is failing and the cash reserve is falling.
Wrong.... because falling cash reserve is the cause of economic failure, however sentence implies that both the facts are of equal importance.

Correct >>> The Economy is failing as the cash reserve is falling.


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