How to use commas and semi-colons
A comma marks a slight break between different parts of a sentence. Used properly, commas make the meaning of sentences clear by grouping and separating words, phrases, and clauses.
It can be useful to think of commas marking places where the reader can pause and draw breath to make sense of an idea.
Below are some ways commas can be used.
1) To separate words in a list
She bought two books, a pen, three notebooks, and a diary.
2) To set off the introductory part of a sentence
First of all, you need to drink regularly to avoid dehydration. Therefore, it is essential to carry a bottle of water with you. After interviewing the patient, make sure to check your notes. If you are not sure about this, please let me know. Although the weather was bad, they decided to go for a walk.
Note: a comma is not necessary here:
Make sure to check your notes after interviewing the patient. Please let me know if you are not sure about this. They decided to go for a walk although the weather was bad.
3) To set off non-essential words and phrases
Incorrect: The man knowing it was late hurried home.
Correct: The man, knowing it was late, hurried home.
* Remember to add the comma before AND AFTER the nonessential word or phrase.
Note: if something or someone is sufficiently identified, the description that follows is considered non-essential and should be surrounded by commas.
Paul, who has a limp, was in a car accident. (If we already know which Paul is meant, this ‘extra’ description is not essential information.)
The boy who has a limp was in a car accident. (We do not know which boy is meant unless we have the description ‘who has a limp’ and, therefore, no commas are used.)
Note the difference in the following sentences:
The 2014 results, which weren’t disclosed immediately, were surprising. (Here we need commas because ‘which weren’t disclosed immediately’ is non-essential, ’extra’ information’)
The results which were achieved in the 2014 trial were
surprising. (Here ‘which were achieved in the 2014 trial’ is essential as it defines which results were surprising. So, no commas are needed.)
Like commas, semicolons indicate an audible pause, slightly longer than a comma but short of a period’s full stop. They can be used in the following ways:
1) To link sentences that are closely related
Here, the semicolon replaces the full stop if the writer wishes to narrow the gap between two closely related sentences. A comma would not be enough here, however, as the semi colon is connecting two ‘stand-alone’ sentences.
Call me tomorrow; you can give me your answer then. We have paid our dues; we expect all the privileges listed in the contract.
2) Before words such as ‘however’, ‘therefore’, ‘that is’ when they introduce a complete sentence.
The weather is improving; however, it is still cold.
He could not understand the question; therefore, he decided to ask for clarification.
(You can also use a full stop in place of the semi-colon here. Note the use of the comma AFTER ‘however’.)
3) To separate long items in a list introduced by a colon
There are several advantages to the paperless office: there would be less need for storage space; money could be saved on buying paper; fewer trees would need to be cut down; and energy would be saved.
4) To separate units of a series when one or more of the units contains commas
Incorrect: The conference has people from Edinburgh, Scotland, Paris, France, London, England and other places as well.
Correct: The conference has people from Edinburgh, Scotland; Paris, France; London, England; and other places as well.
5) To link sentences that are in opposition to each other
His research method was fundamentally flawed; nonetheless, he collected the data.
The Internet Grammar of English
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