English Grammar Review
Clauses and Punctuation

Using the Comma

The comma has two main functions: it isolates nonessential expressions that inform but interrupt, and it separates sentence elements to clarify the relationship between them. Two commas are typically used to isolate expressions; one comma is used to separate elements.

Commas that isolate

We use commas to isolate, or "set off", expressions or clauses that are not essential to the meaning of the sentence.

Question: How do I know that an expression or clause is nonessential?

Answer 1: Remove the expression or clause and decide if the meaning of the sentence changes significantly.

Examples:

We should get the advice of someone who has the proper expertise before opening the computer.

We should get the advice of someone before opening the computer.
[Someone could be anyone, which makes a significant difference in the meaning of the sentence.]

We should get the advice of someone who has the proper expertise before opening the computer.
[essential, so use no commas]

We should get the advice of Ms. Chen who has the proper expertise before opening the computer.

We should get the advice of Ms. Chen before opening the computer.
[It's likely the readers know Ms. Chen, which likely means they already know she has the proper expertise. So, the missing clause would not be essential.]

We should get the advice of Ms. Chen, who has the proper expertise, before opening the computer.
[nonessential, so use commas]

Answer 2: Notice how you would say the expression out loud. If your voice naturally drops, it is not essential. If your voice naturally rises, it is essential.

Qiang would prefer therefore to wait one day.
[voice naturally drops]
[nonessential, so use commas]
Qiang would prefer, therefore, to wait one day.

Qiang would therefore prefer to wait one day.
[voice naturally rises]
[essential, so use no commas]
Qiang would therefore prefer to wait one day.

Commas that separate

When you write a compound sentence (two independent clauses joined by a coordinating conjunction), place a separating comma before the conjunction.

Jin received an email from someone in the Guangzhou office, and she is now getting ready to leave.

When you write a sentence with a compound predicate (the action part of the sentence), do not place a separating comma before the conjunction.

Jin received an email from someone in the Guangzhou office, and is now getting ready to leave.
[wrong]

Jin received an email from someone in the Guangzhou office and is now getting ready to leave.
[right]

When you write a compound sentence where one of the independent clauses is short, you can often safely omit the comma. How short is short? Let's say four words.

Beijing is dry and Guangzhou is humid.
[short then short]

Consider joining and see whether our service is what you need.
[short then long]

See whether our service is what you need and consider joining.
[a little bit long then short]

Commas that separate a series

Using commas in a series usually includes the conjunction and after the final comma.

Wei receives emails from buyers, suppliers, and partners.
[nouns]

After lunch, Qiang returned, sat, and relaxed.
[verbs]

Jin found examples, assembled a list, and reported back.
[predicates]

However, a series of adjectives is often typed with no final conjunction.

Ying is a diligent, thorough, knowledgeable worker.
[adjectives]

Lesson: Using the Comma
Module: Clauses and Punctuation
Course: English Grammar Review