What is a Clause?
"Clause" is a term in grammar denoting a single construction containing a subject and a predicate. In this basic sense a clause is the same thing as a "simple sentence" and is subject to the same difficulty of precise definition.
Some grammarians use the term "clause" for word groups containing some form of the verb other than one required for a predicate, and speak of "infinitive clauses," "participial clauses," and "gerund clauses," but the more general practice is to call these phrases. Since the idea of the clause is most useful for describing sentences that contain more than one clause, some grammarians use the term only in that context.
As parts of a sentence, clauses have the character either of a main clause (also called independent clause, head clause, coordinate clause, or principal clause) or of a subordinate clause (also called dependent clause, subclause, or included clause). A main clause is the essential matrix or leading utterance of the sentence and is not an expansion of any smaller grammatical unit (such as an adjective, noun, or adverb). A subordinate clause either expands a smaller unit, is the equivalent of a smaller unit, or both. Examples of subordinate clauses are:
He likes the boat that I bought yesterday.
He understood what had been done.
When she saw the mouse, she jumped.
Unlike a main clause, a subordinate clause can often be replaced by, or serves the function of, a word or phrase identifiable as a certain part of speech. In the first example above, the subordinate clause is the equivalent of an adjective modifying "boat"; in the second example, the subordinate clause can be replaced by a noun or a noun phrase; in the third example, it can be replaced by an adverb. For this reason, subordinate clauses can be classified according to function as adjectival, adverbial, or nominal (noun). Such classification underscores their dependency upon the main clause.
Structural grammarians have contributed largely to understanding clauses by describing the patterns of intonation (that is, of accent, pitch, and pause) that mark clause boundaries in spoken language. The transformational grammarians have demonstrated how a finite number of independent clause patterns (kernel sentences) are involved in generating the infinite number of sentences than can be formed and understood.