Complex Sentence

A sentence with a main clause and at least one subordinate clause is a complex sentence. The subordinate clause modifies the main clause in a complex sentence. If the subordinate clause modifies a noun or pronoun, it is an adjective clause. If the subordinate clause modifies a verb, it is an adverbial clause.  Either a subordinating conjunction or a relative pronoun separates the main clause from the subordinate clause.

In this complex sentence:

If you want to improve your reading comprehension and writing skills, then you should learn how to conduct a sentence analysis.

The subordinating conjunction is, if.  The subordinating clause is the entire if-clause, If you want to improve your reading comprehension and writing skills. The main clause that it modifies is, then you should learn how to conduct a sentence analysis. The subordinate clause sets the condition for the main clause.

Consider this complex sentence.

As the hot sun rose high on the fourth day of May, the tension on campus grew higher and hotter

The subordinate conjunction is, as. It connects the subordinate clause, as the hot sun rose high on the fourth day of May, to the main clause, the tension on campus grew higher and hotter. (In this case, the connection creates a simile)

The subordinate clause describes when the tension on campus grew hotter and higher, thus, the clause acts as an adverbal clause.