A “clause” contains a subject and predicate like a sentence. And a sentence is, indeed, a clause. But where a sentence is an idea complete in itself, a clause proper may or may not be complete in itself. If there are more than one clause, there will be either coordination or subordination of clauses. When two or more clauses are “main” the clauses are said to be “coordinate.” When a clause begins with a subordinate conjunction, this clause becomes subordinate to a “main” clause. The main clause will be called the “main” clause and other clauses will be called either “subordinate” clauses, since they are subordinate to the main clause, or “dependent” clauses, since they depend on another clause.
A clause is subordinate to a main clause, when it is unable to be an idea “complete in itself.” The sentence, “John drinks beer,” is an (pleasant) idea complete in itself, since there are no terms depending on anything else outside the sentence. But, “John drinks beer, when…” begs the question, “‘When’ what?” Thus the sentence is not complete in itself at this stage. If a clause begins, “When John drinks beer,” then this by itself is not complete in itself either, seeing that we are expecting another clause to “resolve” the clause by giving an answer to the question, “…then what?”
Because these clauses are incomplete, they are called, subordinate clauses or dependent clauses.
These subordinate clauses are typically introduced by a “conjunction.” The word, “conjunction,” is from the Latin “con,” meaning, “with” or “together,” and “junction” from the verb, “jungere,” “to join.” The conjunction thus joins together two words or clauses.
A main clause is an idea complete in itself and, so, may be a sentence, but the subordinate clause can not be a sentence because it requires another clause to complete the idea, of which it is a part.
Of subordinate clauses there are many types. Some conjunctions introduce clauses of time; exempli gratia, “when, while, before, after, during, until.” Eg, “When he drinks beer, John feels good.”
Some subordinate clauses don’t take a conjunction but a relative pronoun. The relative pronoun represents a noun in the preceding clause, which is called the “antecedent.” The clause , which a relative pronoun introduces, is called a “relative clause.” Eg, “Plato taught Aristotle, who started the Lyceum.” “Aristotle” is the antecedent of the pronominal subject of the relative clause, “who.”
Clauses give us more meaning. Clauses may tell us where, when, in what circumstances something occurs. If we think of a painted portrait, the subject may be interesting enough, but to paint the attendant circumstances around him will give us more information. And to do it well will make the picture much brighter or darker, more or less elaborate, more or less emphatic to really carry not just information but much more meaning. In Speech clauses give attendant circumstances, so to more interestingly and informatively paint word pictures.