Commas are not strong enough to join sentences.
Do not use commas to join sentences. Sentences joined by commas are called comma splices. In American English these are generally not acceptable.
To learn more, see:
A pair of commas sets off non-restrictive modifiers and appositives.
A pair of commas acts like tiny parentheses to set off non-restrictive* modifiers, words or phrases that rename or define a noun.
*Restrictive versus non-restrictive
Restrictive elements are necessary to the full meaning of the sentence.
For example:
My brother Bob is a dentist. (My brother Tom sells shoes.)
You have two or more brothers, so the name is needed to restrict, or define, or distinguish, which brother you mean.
Non-restrictive elements (appositives) can be left out of the sentences without a loss of meaning.
For example:
My brother, Bob, is a dentist.
You have only one brother, so his name is not needed to distinguish, define, or restrict, which brother you are talking about.
Another example:
Bella has been going out with her boyfriend, Edward, who is a vampire, for six amazing months.
(She has one boyfriend and he is a vampire forever.)
Versus
Bella has been going out with her boyfriend Edward who is a vampire for six amazing months.
(She has two or more boyfriends and one is a temporary vampire.)
To learn more, see:
Use commas to separate interrupters.
The most common interrupters are:
To learn more, see:
Coordinating conjunctions need a comma before them to join independent clauses.
A coordinating conjunction is a word used to join two complete sentences (independent clauses).
The words most commonly used as coordinating conjunctions are for, and, but, or, yet, and so. An acronym for the most common conjunctions is FANBOYS (for, and, nor, but, or, yet, and so). If you use one of these, ask yourself whether you are joining sentences with it. If you are, add the comma.
These words work with a comma like the two tubes of epoxy glue: you need both to "glue" the sentence together.
To learn more, see:
Use a comma to separate parallel (or independent) adjectives.
Parallel adjectives modify a noun equally. If the adjectives are parallel, you could put the word and between them.
For example:
If a guy is tall and handsome and nice, he is . . .
a tall, handsome, nice guy.
If the guy is tall and handsome and fixes computers, then he is . . .
a tall, handsome computer guy.
(You would not call him a tall and handsome and computer guy. He might be a tall computer guy, or a handsome computer guy, but he is not a tall, handsome, computer, guy.)
To learn more, see:
Use commas to separate and group items in a series.
Serial commas "herd" items or ideas into meaningful groups.
Using the "Oxford comma" (the comma before the and) can eliminate confusion. Using it habitually is easier than deciding if you need it. APA style always uses a comma before the word and at the end of a series.
Commas can change the meaning of a sentence.
Example:
We heard from the lawyers, Richard and Gertrude.
(The lawyers are named Richard and Gertrude.)
We heard from the lawyers, Richard, and Gertrude.
(Richard and Gertrude responded in addition to the lawyers.
To learn more, see:
Commas separate direct quotes or speech from the rest of the sentence.
Use commas after an introductory tag phrase (Jim said, "I'm on my way.") or before an identifying tag phrase (I'm on my way," said Jim.)
Notice the comma goes inside the quotation marks if it is separating the end of a quote from the tag.
To learn more, see:
Use commas to set off absolute phrases, adding detail and variety to your writing.
Commas can also set off absolute phrases, phrases that modify a whole sentence or clause, adding detail, creating complexity, bringin more information into one sentence.
To learn more, see:
Use a comma to set off an introductory phrase or clause or a tag question.
Knowing where the "real" sentence begins or ends a reader can make sense of the rest of the sentence.
Knowing where the "real" sentence begins or ends, a reader can make sense of the rest of the sentence.
More examples:
Learn to spell people.
versus
Learn to spell, people.
Knowing that I felt safe about my parachute jump.
versus
Knowing that, I felt safe about my parachute jump.
No, thanks to you!
versus
No thanks to you!
To learn more, see: