I walk the dog. I go to school.
These are two independent clauses. Right now they are of equal importance. If you join these two independent clauses using one of the FANBOYS, the two clauses will still have equal importance:
I walk the dog, and I go to school.
I go to school, and I walk the dog.
These are compound sentences. In a compound sentence, both independent clauses are of equal importance.
However, when you join two independent clauses into a complex sentence, the ideas are no longer equal. The independent clause is now more important. The subordinate clause is less important.
Subordinate has sub at its root. Sub means below, as in the word submarine (below water). In a complex sentence, the subordinate clause is “below” the independent clause. The subordinate clause is less important.
If you want the clause I walk the dog to be the important idea, keep that as an independent clause and make the other clause subordinate by starting with a subordinate clause word:
I walk the dog before I go to school.
Before I go to school, I walk the dog.
I go to school is now a subordinate clause because it starts with the subordinating conjunction before.
If we want to put emphasis on I go to school, make that the independent clause:
I go to school after I walk the dog.
After I walk the dog, I go to school.
Now I walk the dog is subordinate – less important – because it starts with the subordinating conjunction after.
When two thoughts are equally important, join them into a compound sentence. When one thought is more important than the other, use a complex sentence. The more important idea will be the independent clause. Start the less important clause with a subordinate clause word to make it subordinate.