Before we get into the specifics, we will need to identify two different types of clauses.
Contains information that is essential to the meaning of the sentence. If it were removed, it would change the meaning of the sentence. It is not separated from the main clause by a comma.
A nonrestrictive clause, however, is an adjective clause that adds information that is not needed to determine the identity of the antecedent noun. The meaning of the sentence would not change if it the clause were removed, so it needs separated by a comma.
So, an example of a restrictive clause would be:
The woman who is holding an umbrella will stay dry in the rain. The relative clause, who is holding an umbrella, is restrictive because if it were removed, the meaning of the sentence would change. If you remove it, it doesn’t make sense. The woman will stay dry in the rain. Okay, how is that possible? Is she a witch? You get the idea. So, since the clause is essential, it is a restrictive clause and does not need commas.
The cat who is fastest will eat first.
The person who dies with the most toys wins.
The person who is the whiniest gets taken care of first.
You can see here that if we removed the bold portions, these sentences would not make sense. They are restrictive clauses, so no commas.
Examples of a nonrestrictive clause:
My brother, who is unlikely to ever study for an exam, will probably fail the test.
The new employee, who was hired earlier this week, doesn’t know what he’s doing.
The clean car, that I washed this morning, looked shiny in the afternoon sun.
As you can see, if we were to remove the bold parts, the sentences still make sense. That is what determines if a clause is restrictive or nonrestrictive-if its removal changes the meaning of the sentence.
Thanks for misadventuring with me down the rabbit-hole of English Grammar.
Next post will be comma splices.