Using commas to separate items in a list

EXAMPLE:  Saturday morning started with a hearty breakfast of scrambled eggs, bacon, sausage, and French toast.

Using commas before ‘and’

Note in the example above the use of comma placed before 'and' at the end of a list. This is sometimes known as the Oxford comma.  Some publishers advise against the use of commas before 'and' suggesting it is grammatically incorrect.  For that reason, it is best in general, to avoid its use.  However, the Oxford comma can be used to avoid ambiguity in certain sentences. 

  • EXAMPLE (of useful Oxford comma):  The family meal was soup, fish and chips, and ice cream.

A comma can also be used before an ‘and’ when two statements are linked.

  • EXAMPLE: Charlie has been learning the clarinet for 12 years, and today he is an accomplished member of the local orchestra.

Using commas to separate clauses

Commas are used to separate clauses or parts of a sentence.   

  • EXAMPLE:  Having had lunch, we went back to work.


A ‘restrictive relative clause’ is a part of the sentence that contains information essential to the meaning of the sentence.

  • EXAMPLE: Passengers        who have young children        may board first


If you left out the clause 'who have young children' the sentence would not make much sense.

EXAMPLE:  Passengers may board the aircraft first. (This sentence does not make sense).


You should not put commas around a restrictive relative clause.

  • AVOID: Passengers, who have young children, may board first (these commas are not necessary)


The other type of subordinate clause beginning with ‘who’, ‘which’, ‘whom’, etc. is known as a ‘non-restrictive relative clause

A non-restrictive relative clause contains information that is not essential to the overall meaning of a sentence.  You need to put a comma both before and after a non-restrictive relative clause.

  • EXAMPLE:  Mary, who has two young children, has a part-time job.  

If you remove this clause, the meaning of the sentence is not affected and it still makes perfect sense.

  • EXAMPLE:  Mary has a part-time job in the library.

Using commas to mark off parts of a sentence

Commas are used to separate a part of a sentence that is an optional ‘aside’ and not part of the main statement.

EXAMPLE:   Gunpowder is not, of course, a chemical compound.

In these sentences, the role of the commas is similar to their function in non-restrictive relative clauses: they mark off information that isn’t essential to the overall meaning. Using commas in this way can really help to clarify the meaning of a sentence. 

  • EXAMPLE:  Cynthia’s daughter, Sarah, is a midwife.

If you aren’t sure whether you’ve used a pair of commas correctly, try replacing them with brackets or removing the information enclosed by the commas altogether, and then see if the sentence is still understandable, or if it still conveys the meaning you intended.

Using a comma with 'however'

Use a comma after 'however' when however means 'by contrast' or 'on the other hand':

  • EXAMPLE:  However, a good deal of discretion is left in the hands of area managers.

Don't use a comma after however when it means 'in whatever way'.

  • EXAMPLE:  However you look at it, existing investors are likely to lose out.

Incorrect use of commas / the ‘comma splice’

A comma splice happens when a comma inappropriately links two independent clauses. The comma splice splices together two clauses that are each complete in their own right.

  • EXAMPLE: She’s an outstanding student, she’ll go far.

The comma here may well represent how people say the two clauses out loud. However, in any formal or academic writing,  it is deemed incorrect.

Three ways of avoiding the comma splice

1. Make the two clauses into separate sentences.
COMMA SPLICE:  Dan was late, we left without him.
EXAMPLE (corrected):  Dan was late.  We left without him.


2. Use a conjunction such as ‘and’ or ‘but’, or ‘as’, ‘because’, ‘so’, if there is a causal connection.
EXAMPLE (corrected):  Dan was late so we left without him.


3. Use a semicolon (;)
EXAMPLE (corrected):  Dan was late; we left without him.


References 

Oxford Living Dictionaries. 'punctuation/comma' Retrieved from https://en.oxforddictionaries.com/punctuation/comma

Oxford Living Dictionaries. 'punctuation/comma splice' Retrieved from https://en.oxforddictionaries.com/grammar/the-comma-splice