Non-essential VS Essential Clauses (and commas)
What does "essential" mean here? It means the clause is needed to help the reader know exactly what you are talking about.
We need the adjective clauses in these two sentences to IDENTIFY which man teaches science and which one teaches math, right?
Check it out.... If you take the adjective clause out of these sentences, can you still tell me WHICH MAN is the science teacher and which man is my math teacher?
In these sentences, the adjective that we took out was essential to the sentence because it told us "which man" we are talking about. It helped us identify that man.
What does "non-essential" mean? It means that the adjective clause is NOT needed in order to IDENTIFY the referent (what we are talking about). It is simply additional information.
Look at the non-essential clause below. The information about the hats is NOT essential to know which "Mr. Smith" or which "Mr. Jones" we are talking about.
Double-Check: Can we understand who we are talking about, even if we eliminate the adjective clauses?
Yes we can! This means the clause is non-essential / non-restrictive.