The comma is used for clarity in separating different parts (words, phrases, or clauses) of a sentence.
e.g. The bag contained old shoes, worn clothes, and a pair of trousers.
The comma before and is optional, but is preferable where clarity may be an issue. The comma is not omitted before and in a series of independent clauses.
e.g. The man took the key, his children carried the box, and their dog followed them.
The comma separates independent clauses joined by a coordinating conjunction (but).
e.g. This is an excellent book, but many have not read it.
The comma separates a dependent clause from an independent one.
e.g. Although this is an excellent book, many have not read it.
The comma separates coordinate adjectives (describing the same noun) without the conjunction and.
e.g. a tall, dark, handsome man (coordinating adjectives)
However, the comma is omitted in cluster adjectives (describing the subsequent words)
e.g. a dark brown leather jacket (dark describes brown; brown describes leather; and leather describes jacket)
The comma is used for clarity of meaning.
e.g. At sixty-five, you may consider retirement.
e.g. Not getting any sleep, she felt exhausted.
e.g. To write effectively, you must learn some basic writing skills.
The comma separates a non-essential clause or sentence element from the rest of the sentence.
e.g. Look at this book, which was found on the kitchen floor!
There is only one book here, and it was found on the kitchen floor; which was found on the kitchen floor becomes only additional but not essential information (indicated by the presence of the commas).
Look at another example:
e.g. Look at this book that was found on the kitchen floor!
There are many other books, and this one was found on the kitchen floor; that was found on the kitchen floor is essential information because it identifies which book to look at (indicated by the absence of the commas).
The comma separates modifiers and conjunctive adverbs.
e.g. on the other hand,
e.g. for example,
e.g. in fact,
e.g. in the first place,
The comma is NOT used before subordinating conjunctions (after, although, because, before, if, since, unless, until, when, where).
e.g. You cannot leave now because the airport is closed. (NO comma)
e.g. Because the airport is closed, you cannot leave now. (comma here)
e.g. Do not call 911 unless it is an emergency. (NO comma)
e.g. Unless it is an emergency, do not call 911. (comma here)
e.g. We left the bar when we finished our drinks. (NO comma)
e.g. When we finished our drinks, we left the bar. (comma here)
Copyright© by Stephen Lau