A conjunction joins words and groups of words.
There are two classes of conjunction: co-ordinate or coordinating and subordinate or subordinating.
Co-ordinate conjunctions: and, but, either…or, neither…nor.
Subordinate conjunctions: that, as, after, before, since, when, where, unless, if.
Mother and Father are driving me to New Orleans. (and is a coordinate conjunction joining words of equal significance in the sentence.
I painted the walls but Jack painted the woodwork. (but is a coordinate conjunction joining clauses of equal significance in the sentence. Either clause could stand alone as a sentence.)
Since you can’t get away, we’ll go without you.
(Since is a subordinate conjunction joining a less important thought to a more important thought. The main clause, we’ll go without you, can stand alone as a complete thought. The subordinate clause, Since you can’t get away, is an incomplete thought. It is dependent upon the main clause for meaning.)
NOTE: The relative pronouns who, whom, which, and that are used in the same way that subordinate conjunctions are. The difference is that the relative pronouns serve three purposes at once:
1) they stand for a noun in the main clause
2) they connect the clauses
3) they serve as a subject or object word in the subordinate clause:
He is the man who invented the hula hoop. (who stands for man and is the subject of invented)
Charles is the boy whom the other children tease. (whom stands for boy and is the object of tease)
Give me the piece of string that is waxed. (that stands for string and is the subject of is waxed)
There goes the horse which won the Derby. (which refers to horse and is the subject of won)
The possessive adjective whose can also be used to join clauses:
That’s the bird whose plumage I admire. (whose refers to bird and describes plumage)