Introductory & Subordinate Clauses

31. When Do I Use Commas?

B. To join introductory clauses, phrases, or subordinate clauses that come before the main clause

I. An introductory clause or phrase acts as a qualifying or clarifying statement about the main sentence. Think of it as an addition to the main thought of a sentence. The introductory language is usually a dependent statement (see section A above). In order to indicate to the reader where the main thought begins and ends, and thus clarify the writer's point, a comma should be used after the introductory language. Again, these words are often contained in a dependent phrase anyway.

• Example: “By 1700 BC, wheat could not be grown in Mesopotamia due to salinization of the soil."

This sentence has both an introductory clause, “By 1700 BC,” and a dependent clause, “due to salinization of the soil.” The main sentence, “wheat could not be grown in Mesopotamia,” is being amended or clarified by these clauses.

II. Phrases are commonly used as introductions to a sentence. Words like 'however,' 'indeed,' 'therefore;' phrases like 'on the one hand,' 'in particular,' 'for example,' and 'in the meantime' are examples of introductory language that ought to be concluded with a comma when they begin a sentence.

• Example: "However, the Fertile Crescent may not actually have been as fertile as the name suggests."

III. A clause can be “tested” for its function by moving the language in question to the end of the main clause and asking, "does it still work in the sentence?"

• Example: “The Fertile Crescent may not have actually been as fertile as the name suggests, however.”

Notice here that the introductory word was moved to the end of main sentence in order to test its function. Because it still makes the point and clarifies the main sentence in this new position, we can determine that it is an introductory phrase and thus that it ought to be concluded with a comma if it is placed at the beginning of the main sentence, or preceded by a comma when it is placed at the end.

• Example: “Wheat could not be grown in Mesopotamia by 1700 BC due to salinization of the soil.”

Notice that this looks like a compound sentence with a dependent clause at the end—like the example from Section A.IV above (it actually has two dependent clauses tacked on—“by 1700 BC” and “due to salinization of the soil”). The introductory phrase needs a comma if it will be placed at the beginning of the main thought, but may not if it is placed at the end. This is because it can function as either introductory language or an essential statement (see below). If it comes later in the sentence, the conjunction will suggest whether to use a comma or not.

If the introductory sentence, when placed at the end of the main clause, no longer properly introduces, clarifies, or amends the main clause, you may have two independent clauses. Follow the rules from section A above for dealing with independent clauses.

• Example: “The ancient Mesopotamians developed a canal system, and the soil in the area became severely salinized.”
• Example: “Mesopotamian soil became severely salinized, and the ancient Mesopotamians developed a canal system.”

In the second example, the two independent clauses indicate cause and effect and so cannot be reversed without destroying the meaning of the sentence. Hence neither functions as a dependent or introductory clause, and the comma usage must follow the rules governing compound sentences.

IV. Watch out for sentences that start with introductory statements, but then introduce a main clause that looks like a serial list. This is precisely the type of error the Oxford comma is designed to fix, but it means that special attention must be paid to places where the Oxford looks like it should be used but isn't.

• Example: “Because of its unique geography, boundaries and inhabitants tended to be transient in southern Mesopotamia.”

Directly following the comma behind "geography" are two more objects--boundaries and inhabitants--which leads the reader to think a serial list is coming. Thus I might have to stop reading at "tended" because I was expecting a verb that relates to the entire series in the list I thought I just read (something that speaks to all of "geograpy, boundaries, and inhabitants"). Because it doesn't, I have to back up, reread the sentence, and understand it in a different way--the way you actually meant it. But this process confuses the point and puts your style in front of your argument. However, the only way to fix this issue is to change the sentence--changing or removing a comma will only turn the sentence into something else and cause additional problems.

• Example: “The boundaries and inhabitants of southern Mesopotamia tended to be transient because of the area's unique geography.”

By seperating the subject (boundaries and inhabitants) from the object (geography), you now don't have the issue with list confusion. Since commas are the problem in this type of sentence structure, adding or removing them won't help. But fixing the structure does, and it's an easy fix to apply.

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I.	An introductory clause or phrase acts as a qualifying or clarifying statement about the main sentence. Think of it as an addition to the main thought of a sentence. The introductory language is usually a dependent statement (see section A above). In order to indicate to the reader where the main thought begins and ends, and thus clarify the writer's point, a comma should be used after the introductory language. Again, these words are often contained in a dependent phrase anyway.
	• Example: “By 1700 BC, wheat could not be grown in Mesopotamia due to salinization of the soil."
	This sentence has both an introductory clause, “By 1700 BC,” and a dependent clause, “due to salinization of the soil.” The main sentence, “wheat could not be grown in Mesopotamia,” is being amended or clarified by these clauses.

II.	Phrases are commonly used as introductions to a sentence. Words like 'however,' 'indeed,' 'therefore;' phrases like 'on the one hand,' 'in particular,' 'for example,' and 'in the meantime' are examples of introductory language that ought to be concluded with a comma when they begin a sentence.
	• Example: "However, the Fertile Crescent may not actually have been as fertile as the name suggests."

III.	A clause can be “tested” for its function by moving the language in question to the end of the main clause and asking, "does it still work in the sentence?"
	• Example: “The Fertile Crescent may not have actually been as fertile as the name suggests, however.”
	Notice here that the introductory word was moved to the end of main sentence in order to test its function. Because it still makes the point and clarifies the main sentence in this new position, we can determine that it is an introductory phrase and thus that it ought to be concluded with a comma if it is placed at the beginning of the main sentence, or preceded by a comma when it is placed at the end.
	• Example: “Wheat could not be grown in Mesopotamia by 1700 BC due to salinization of the soil.”
	Notice that this looks like a compound sentence with a dependent clause at the end—like the example from Section A.IV above (it actually has two dependent clauses tacked on—“by 1700 BC” and “due to salinization of the soil”). The introductory phrase needs a comma if it will be placed at the beginning of the main thought, but may not if it is placed at the end. This is because it can function as either introductory language or an essential statement (see below). If it comes later in the sentence, the conjunction will suggest whether to use a comma or not.
	If the introductory sentence, when placed at the end of the main clause, no longer properly introduces, clarifies, or amends the main clause, you may have two independent clauses. Follow the rules from section A above for dealing with independent clauses.
	• Example: “The ancient Mesopotamians developed a canal system, and the soil in the area became severely salinized.” • Example: “Mesopotamian soil became severely salinized, and the ancient Mesopotamians developed a canal system.”
	In the second example, the two independent clauses indicate cause and effect and so cannot be reversed without destroying the meaning of the sentence. Hence neither functions as a dependent or introductory clause, and the comma usage must follow the rules governing compound sentences.
IV.	Watch out for sentences that start with introductory statements, but then introduce a main clause that looks like a serial list. This is precisely the type of error the Oxford comma is designed to fix, but it means that special attention must be paid to places where the Oxford looks like it should be used but isn't.
	• Example: “Because of its unique geography, boundaries and inhabitants tended to be transient in southern Mesopotamia.”
	Directly following the comma behind "geography" are two more objects--boundaries and inhabitants--which leads the reader to think a serial list is coming. Thus I might have to stop reading at "tended" because I was expecting a verb that relates to the entire series in the list I thought I just read (something that speaks to all of "geograpy, boundaries, and inhabitants"). Because it doesn't, I have to back up, reread the sentence, and understand it in a different way--the way you actually meant it. But this process confuses the point and puts your style in front of your argument. However, the only way to fix this issue is to change the sentence--changing or removing a comma will only turn the sentence into something else and cause additional problems.
	• Example: “The boundaries and inhabitants of southern Mesopotamia tended to be transient because of the area's unique geography.”
	By seperating the subject (boundaries and inhabitants) from the object (geography), you now don't have the issue with list confusion. Since commas are the problem in this type of sentence structure, adding or removing them won't help. But fixing the structure does, and it's an easy fix to apply.