A principal clause is one that does not depend on any word.
A subordinate clause is one that depends upon some word or words in the principal clause.
We have found, also, that these principal clauses are always connected by co-ordinate conjunctions, for they are of equal rank and importance; neither is dependent upon the other.
Subordinate clauses are always connected with the principal clause by a subordinate conjunction. The subordinate clauses which we have been studying have all been adverb clauses which are used to describe the action expressed in the verb contained in the principal clauses.
The subordinate clause in a sentence may also be used as a noun. When the subordinate clause is used as a noun it is called a noun clause.
362. A noun clause is a clause used as a noun.
A noun clause may be used in any way in which a noun is used, except as a possessive. It may be used as a subject, an object, a predicate complement, or in apposition with a noun. These noun clauses may be introduced by either relative pronouns, interrogative pronouns or by conjunctions. For example:
In the first sentence, who he is, is a noun clause used as the object of the verb know. It tells what I know, and is the object of the verb know,—just as if I had said; I know the facts. In this sentence the noun, facts, is the object of the verb know.
In the second sentence, He asked, "what do you want?" the noun clause what do you want is the object of the verb asked, and is introduced by the interrogative pronoun what.