Comma

Commas are used to separate items in a series or list.

For example:

French, Spanish, Italian and Portuguese descend from Latin.

Note that there is no comma before the last item on the list, ‘Portuguese’.  In some cases, however, a comma before the last item may avoid confusion, as in this example:

I have read Emma, Sense and Sensibility, and Pride and Prejudice.

Two of the three novels on the list contain the word ‘and’ in the title. In this case, the use of a comma before ‘and Pride and Prejudice’ avoids confusion by keeping the titles separate.

Using a comma before the last item is also useful when the items listed are long:

Writing a literature review involves searching the literature, critically evaluating the contribution of each study, and identifying gaps in the existing research.

Commas are used between adjectives that can be joined by ‘and’.

For example:

This is a well-written, beautifully-illustrated book.

You could also say: ‘This book is well written and beautifully illustrated’, so the use of the comma is appropriate in this case.  

However, do not use a comma if the adjectives cannot be joined by ‘and’, as in this example:

He insisted on buying an expensive mobile phone.

Since we cannot say ‘an expensive and mobile phone’, or ‘a phone that is expensive and mobile’, there should be no comma between ‘expensive’ and mobile’.

Commas are used after connectors that link a sentence to the previous one.

For example:

There has been a significant increase in the price of food. Therefore, it is likely that inflation will rise in the next quarter.

There has been a great deal of research into bilingualism. However, little is known about the acquisition of a third or fourth language.

Connectors joining a sentence to the previous one may also occur in mid-position in the second sentence. In this position, connectors are both preceded and followed by a comma:

There has been a significant increase in the price of food. It is likely, therefore, that inflation will rise in the next quarter.

There has been a great deal of research into bilingualism. Little is known, however, about the acquisition of a third of fourth language.

Avoid run-on sentences

It is not unusual to see occurrences of ‘run-on’ sentences in student writing. In a ‘run-on’ sentence, two separate ideas, each constituting a complete grammatical unit (or independent clause), are joined with no punctuation or conjunction. Sometimes, a comma is used to separate the two clauses. This is also incorrect. Here are two examples:

Incorrect: The disease has now spread from densely-populated urban areas to isolated rural villages this gives cause for concern because there are few medical facilities in these remote areas.

Incorrect: The disease has now spread from densely-populated urban areas to isolated rural villages, this gives cause for concern because there are few medical facilities in these remote areas.

Since the examples above contain two separate ideas, and each is in a grammatically complete unit, a full stop is needed after the word ‘villages’. The correct punctuation is:

Correct: The disease has now spread from densely-populated urban areas to isolated rural villages. This gives cause for concern because there are few medical facilities in these remote areas.