- Demonstrate the standard uses of commas as separators
Commas before and after transition words help to separate them from the sentence ideas they are describing. Transition words tend to appear at the beginning of a sentence but they can also be found in the middle or at the end, as in the following:
- Therefore, the natural gas industry can only be understood fully through an analysis of these recent political changes.
- The lead prosecutor was prepared, however, for a situation like this.
- The use of commas does not need to be difficult, however.
When transition words appear between two independent clauses (or two sentences that could stand alone), however, a period or semicolon is required beforehand:
- Clint had been planning the trip with his kids for three months; however, when his boss called and asked him to work, he couldn’t say no.
- Sam was retired. Nevertheless, he wanted to help out.
As you can see from these examples, a comma is always required after a transition word.
The key here is to ask yourself whether the transition word connects two sentences.
- The food, however, is delicious.
“The food” is not a complete sentence nor is “is delicious.”
- The food is delicious; however, I am full.
- The food is delicious. However, I am full.
“The food is delicious” is a complete sentence. “I am full” is a complete sentence.
Descriptive phrases add extra information about the things that they describe and need commas.
- Near the end of the eighteenth century, James Hutton introduced a point of view that radically changed scientists’ thinking about geologic processes.
This is extra information about when James Hutton introduced this point of view.
- James Lovelock, who first measured CFCs globally, said in 1973 that CFCs constituted no conceivable hazard.
This is extra information about James Lovelock.
- All of the major industrialized nations approved, making the possibility a reality.
This is extra information about what the approval means.
In each example, the phrase separated by the comma could be deleted from the sentence without destroying the sentence’s basic meaning. If the information is necessary to the primary sentence meaning, it should not be set off by commas.
- My sister, who is beautiful and brilliant, is ten years older than I am.
The phrase “who is beautiful and brilliant” gives more information about my sister and so we can think about enclosing it in a pair of commas (almost like parentheses).
But let’s take a look at a variation on this sentence.
My sister, who is ten years older than I am, is beautiful and brilliant.
The phrase “who is ten years older than I am” is only extra information if I have just one sister. If I have more than one sister, then we need to remove the commas because the phrase within explain which sister I am talking about.
This is a pretty specific and tricky rule, but it is often on standardized tests.
Here is another example.
- I have three sisters and one brother.
Because I have several sisters, the name “Susan” is not extra information, so we do not use commas. Because I have only one brother, his name is extra information. Let’s look at two more examples:
- My sister Susan is a great baseball player.
- My brother, Ilton, is a fantastic piano player.
Because I have several sisters, the name “Susan” is not extra information, so we do not use commas. Because I have only one brother, his name is extra information.
Another way to think about this is that the information within the commas needs to be removable.
- My sister, Susan, is a great baseball player.
- My sister is a great baseball player.
If I have more than one sister, the second version doesn’t make sense. You can check the example by asking which sister is being referred to in this sentence. So, the commas are incorrect.
Commas with Dates and Locations
Consider the following sentence:
- The river caught fire on July 4, 1968, in Cleveland, Ohio.
We use commas with dates (July 4, 1968) and between a city and state in the United States (Baltimore, Maryland). This also applies to cities and other countries.
- Paris, France, is beautiful this time of year.