Determine whether the element is restrictive or nonrestrictive.
For example, compare the following sentences:
1. The boy sitting on the striped towel is Lisa's son.
2. John, who likes to chew gum, is Lisa's son.
In the first sentence, the modifier, "sitting on the striped towel," gives information that is essential to determining which particular boy, of potentially many, is Lisa's son.
In the second sentence, the modifier, "who likes to chew gum," simply adds extra details about John that are not necessary to the message that he is Lisa's son.
Always use commas to separate nonrestrictive elements from the rest of the sentence. If the nonrestrictive element appears in the middle of a sentence, it should have a comma on either side. If the nonrestrictive element is at the end of a sentence it should be introduced by a comma and concluded with a period.
For example:
The club president, who grew up in Idaho, is speaking at the banquet.
I love to go skiing when there is fresh snow, which is rare.
Pay close attention to modifiers that could function as either restrictive or nonrestrictive elements. Make sure to use commas if you intend the element to be interpreted as a nonrestrictive element.
For example, compare the following sentences:
1. The new restaurant that serves burritos is outstanding.
2. The new restaurant, which serves burritos, is outstanding.
In the first sentence, the absence of commas around the modifier tells the reader that the information is essential to determining which particular restaurant, of potentially many, is outstanding.
In the second sentence, the commas around the modifier tell the reader that the information simply adds extra details about the new restaurant and are not necessary to the message that it is outstanding.