The comma has a variety of uses. It separates words and clauses to produce a piece of writing that can be read more efficiently.
Replacing ‘And’
The comma replaces the word ‘and’ in separating a series of words belonging to the same part of speech, whether nouns, adjectives, verbs or adverbs:
‘I saw Diane, Jo and Kate.’
‘The car slowed down, stalled and stopped.’
‘He made conversation cheerfully, wittily and interestingly.’
‘He was an extrovert, restless and ambitious man.’
The word before ‘and’ takes a comma only in certain instances, for example:
‘Her favorite fashion brands are DKNY, Versace, and Dolce and Gabbana
Without the comma after ‘Dolce and Gabbana’ it might appear that ‘Dolce and Gabbana’ are two different brands.
Use the Comma to Separate Words
The comma separates words when someone is addressed:
‘Will you, if you have time, please visit us?’
It also separates words and phrases in apposition and participial phrases:
‘Barack Obama, the President of the United State, delivered a speech to Congress.’
‘Having looked at the menu, Dennis chose the lamb stew.’
‘All things considered, I prefer the countryside.’
Separating Clauses
In complex sentences the comma is used where a short pause is required to separate clauses:
‘Unless she amends her ways, she will be asked to leave.’
‘The man, who is a millionaire, visits Monte Carlo twice a year.’
The comma would not, however, be needed if the above sentence read:
‘The man who visits Monte Carlo twice a year is a millionaire.’
In this sentence the adjective clause ‘who visits Monte Carlo twice a year’ defines the subject of the sentence: the man is identified as the one who visits Monte Carlo twice a year. When the adjective clause in a sentence defines the subject, no comma is needed. When the adjective clause merely comments on the subject without affecting its definition, then commas are required to separate the clause from the main subject and the verb. Thus in the sentence above, ‘The man, who visits Monte Carlo twice a year, is a millionaire’, the adjective clause comments on the subject, giving additional information about the man, but does not identify him, as in ‘The man who visits Monte Carlo twice a year…’ When the adjective clause is a non-defining one, commas are necessary.
Separating Two Parts in Double Sentences
The comma is used to separate two parts in double sentences, when the second subject is expressed:
‘He wanted to argue, but the teacher would not allow him.’
If the subject of the two parts is the same, then no comma should be used:
‘He wanted to argue but was afraid of getting into trouble.’
Commas are usually unnecessary with words like ‘therefore’, ‘however’, ‘at last’, etc, except for emphasis:
‘Every student must therefore revise thoroughly.’
This sentence would be made more emphatic if a comma was placed after ‘must’ and ‘therefore’:
‘Every student must, therefore, revise thoroughly.’
Unless you wish to stress the meaning, it is better to omit commas when using ‘however’, ‘therefore’ and similar words.
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