Today, let’s visit the world of parenthetic expressions. This is, when you are writing, a brief pause in the middle of a sentence to add some additional information. It also applies to speaking, but I wanted to give you an example of what I was talking about right up front.
Sometimes it is hard to decide if you need the commas, but, if you have one comma, you must include the second one.
For example, you would not say the following:
This is, when you are writing a brief pause in the middle of a sentence to add some more information.
This looks like someone is just putting a comma after a verb. My students do this frequently; it is incorrect.
Here is the same sentence with no commas:
This is when you are writing a brief pause in the middle of a sentence to add some more information.
See how fast that gets confusing? Therefore, when you have a parenthetic expression, use two commas. Keep me happy.