+347.+ We have found that co-ordinate conjunctions connect words,
phrases and also clauses that are entirely independent; that is, they do
not depend in the slightest degree upon any other word, phrase or
clause. Subordinate conjunctions connect inferior clauses to the main
clauses of the sentence. These inferior clauses are dependent clauses.
Subordinate conjunctions never connect words or phrases; but only
dependent clauses, to the rest of the sentence. Note the following
sentences:
He came _quickly_.
He came _on time_.
He came _when he was called_.
In the first sentence the word _quickly_ is an adverb modifying the verb
_came_ and answers the question _when_. It tells _when_ he came. In the
second sentence, the phrase _on time_ is an adverb phrase modifying the
verb _came_, and answers the question _when_. It tells _when_ he came.
In the third sentence, the clause _when he was called_, also answers the
question _when_, and tells _when_ he came. Therefore, it is a clause
used as an adverb. It is different from the phrase _on time_, for the
phrase _on time_ does not contain a subject and a predicate.
+348.+ The difference between the phrase and the clause is that the
phrase does not contain either a subject or a predicate, while the
clause _always_ contains both a subject and a predicate. So in the
clause, _when he was called_, _he_ is the subject and _was called_ is
the predicate, and _when_ is the subordinate conjunction, which connects
this adverb clause to the verb _came_, which it modifies. The clause _he
came_, and the clause _when he was called_, are not of equal rank and
importance, because the clause, _when he was called_, simply modifies
the verb contained in the clause _he came_, by describing the _time_ of
the action expressed in the verb _came_. So the clause, _when he was
called_, is a subordinate or dependent clause, and the conjunctions
which connect this class of clauses to the main clause are called
subordinate conjunctions.
+349.+ +A subordinate conjunction is one that connects a dependent
clause to the principal clause.+