A simple sentence is equivalent to a single clause and we have already seen how the comma works in a single clause. What we now have to see is how the comma works when there is more than one clause (complex and compound sentences).

A sentence may contain a relative clause.  If the relative clause is one which identifies the antecedent, a comma is used: the girl who sat next to me on the bus. The clause beginning with who only helps identify what girl I am talking about.  In technical language such clauses are called defining clauses. They identify the referent of the antecedent. Observe there is no comma between the girl (the antecedent) and who…(the defining clause.)

As against such clauses, there are relative clauses which give additional information about the antecedent. My mother, who lived to be eighty, had a proverb at the tip of her tongue all the time. The clause beginning with who doesn’t identify my mother. There is no need to do so. It only gives some additional information about her.  Such clauses are called non-defining clauses. They are set apart by commas.

A clause may also explicate the content of a noun.  The news that there were some blasts in Bangalore…. What is the content of the news? ‘That there were some blasts in Bangalore’. Such clauses are called content clauses.  Notice there is no comma separating the head (the news) from the following clause. (Clauses of this type are also called Appositional clauses. ‘Content clause’ is the name given by the great grammarian Otto Jespersen.)
Noun clauses can also function as the subject (That global warming is a fact is now undisputed), object (We know that oil reserves are fast shrinking) and complement (The fact is that he is a moron).  In no case will a comma separate the clause from the rest of the sentence. This follows from the general principle that the elements (+constituents) of a clause cannot be separated by any comma.

It is with adverb clauses that we see the comma coming into full pay.  When the clause naturally occurs at the end, there is no comma. He turned pale when he saw her. But when the clause is in displaced position, a comma appears: When he saw her, he turned pale.

Even when they occur in their normal position (at the end), a comma will be found when the connection between the main clause and the adverbial clause is not explicitly presented. Stand there, that I may look at you. Where the connection is made explicit by a suitable correlative, no comma appears. Stand there so that I may look at you. This is so with other punctuation marks, too. We were tired; we had to halt. We were so tired that we had to halt. He was in such poor health that he resigned.

All adverb clauses are adjuncts. They give the attendant circumstances of the action indicated in the main clause. Their place and rank is after subject, object, and complement. But for rhetorical reasons they may be shifted.  This is so, generally speaking, with clauses of condition. They normally appear in front position and are set apart by commas. If you drop the bowl, it will break. Should he call, tell him I am not at home. 

It is not possible to examine here all adverbial clauses. But the general point should be clear: when moved to the front, with the main clause following, a comma separates the adverb clause. And some clauses, like if-clauses (clauses of condition) are generally in front position.

In the analysis till now (comma at the sentence level with multiple clauses) we have noted that a clause in a displaced position is marked by a comma. A clause in its normal position is also separated by a comma if its structural relation with the previous (usually main) clause is not explicit.

These remarks extend, without qualification, to reduced adverbial clauses as in: Returning home, he found the place burgled (cf. He found the place burgled when he returned home). The authorities arriving, the ceremony began.

(to be concluded)
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