The following lesson is an adapted extract from the book, Advanced Writing Skills for Students of English. I’ve decided to share it here as I’ve had a few questions relating to punctuation and sentence structure lately, and this gives a useful introduction to how commas help signal longer sentences. Commas are typically used to separate clauses in complex sentences, when we have a main clause and one or more subordinate clauses:
- The passengers waited outside, while the steward refused to open the door.
The comma comes at the end of the first clause. In regular sentence structure, the comma often comes before a conjunction or other linking word. When complex sentences are reversed, with the linking word at the start of the sentence, the comma still comes at the end of the first clause.
- While the steward refused to open the door, the passengers waited outside.
Commas in complex sentences are sometimes seen as optional, as the comma often has little impact on the meaning or understanding of the sentence. This can depend on the sentence itself, as commas between clauses are less necessary in shorter sentences or sentences where the clauses are more directly connected:
- They ate the cake before she could stop them.
However, with longer sentences, and sentences where the co-ordinating conjunction might create confusion with another noun, a comma can have a big impact on the meaning. This is particularly true with conjunctions like as and while, which could be interpreted as either for the purposes of or at the same time as:
- The accountant demanded a new lamp, as the dim light made work impossible. (because working was impossible)
- The accountant demanded a new lamp as the dim light made work impossible. (The demand was made at the same time as the light worsened.)
Commas are essential with particular types of subordinate clauses, such as non-defining relative clauses. Relative clauses, separated by the words who, whom, that, and which, may include defining information (with no commas) or additional information (with the clause separated by commas).
- The man who had stolen the mango was imprisoned.
- The man, who had stolen the mango, was imprisoned.
In the above example, the first sentence tells us this particular man (the one responsible for the crime, not someone else) was imprisoned. The second sentence tells us a man was imprisoned who happened to commit this crime. In the first sentence, the relative clause (who had stolen the mango) is used as an identifying cause of imprisonment; in the second sentence, it is additional information, not necessarily the cause of imprisonment (the clause may be confirming that he was guilty, for example). This is a subtle example. How much difference the commas make with relative clauses may vary depending on the situation:
- The team who wanted the prize most won. (They won because of their passion.)
- The team, who wanted the prize most, won. (They won and also had the most passion.)
- Visitors that require special assistance should contact the front (only those who require assistance should contact the desk)
- Visitors, who require special assistance, should contact the front desk. (suggesting all visitors require special assistance)
This use of commas overlaps with considerations for separating additional information within a sentence, which is covered in the another section in the book, which I will upload soon to complement this article. Hopefully this helps as a starter, and if you find such information interesting do check out the whole book!