Video: Comma story - Terisa Folaron, from TedEd (new tab). (Watching time: 4m:59secs). Content: Commas used with conjunctions and subordinates.
Commas in front of conjunctions
A commas goes before a conjunction, unless one of the clauses involved does not have a (grammatical) subject.
Commas are commonly used before a joining word when linking two complete independent (or dominant) sentences. A complete sentence is one that can exist by itself and makes complete sense (... it has its own subject and its own verb). A joining word (conjunction) are words such as and, but, although, or, when and if.
If either of the two phrases within a sentence is not complete (for example it doesn't have its own subject or verb), then no comma is used before the joining word. Look at the examples below to see this demonstrated.
I knew the opposition were good, but I had never seen them play.
I knew the opposition were good but had never seen them play.
You have to be a little careful with this advice. A comma can be ommitted before conjunctions joining two short complete sentences (see example below)...
I walked to town but Mary cycled.
Commas for separating independent and dependent clauses
If you have a dependent clause beginning your sentence, you use a comma to signal where the independent clause begins.
A comma separates a dependent clause from an independent clause.
In order to save New Zealand from bankruptcy, the government is selling its assets at discount prices.
This is fun! Watch the BBC's video and try their three games on compound sentences: BBC lesson on connectives. (Not iPad friendly).
Instructions: Hover or tap the questions below to see the reason for the answers given in the quiz.
1. Select the option that successfully corrects the error in the sentence below.
Over the past few months I've had a problem with drinking I need to take measures to address that.
Over the past few months, I've had a problem with drinking, and I need to take measures to address that. ✔
Why: This lesson explains how a comma is used before a conjunction to link two independent clauses in a sentence. In this case neither clause is subordinate to the other, so there should be a comma separating them.
2. Change the punctuation and add a joining word to make these two sentences into one sentence.
Sentence 1: We have been in a relationship online for 6 years.
Sentence 2: I have never met her in person.
We have been in a relationship online for 6 years, but I have never met her in person. ✔
Why: This grammar is about using commas and conjunctions to make a complex sentence out of two simpler sentences.