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.

Essential clauses

We need the adjective clauses in these two sentences to IDENTIFY which man teaches science and which one teaches math, right?

• The man who is wearing the red hat is my science teacher. 
• The man who is wearing the green hat is my math teacher.

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?

• The man who is wearing the red hat is my science teacher. 
• The man who is wearing the green hat is my math teacher.

• The man  is my science teacher.  (Which man are we referring to? You can't tell because "the man" is not specific.)
• The man is my math teacher. (Which man are we referring to? Are we talking about the same man?  We don't know.)

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. 

Non-Essential Clauses

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. 

• Mr. Smith, who is wearing the red hat, is my science teacher.
• Mr. Jones, who is wearing the green hat, is my math teacher.

• Mr. Smith, who is wearing the red hat, is my science teacher.
• Mr. Jones, who is wearing the green hat, is my math teacher.

Double-Check:  Can we understand who we are talking about, even if we eliminate the adjective clauses?

• Mr. Smith is my science teacher.
• Mr. Jones is my math teacher.

Yes we can!  This means the clause is non-essential / non-restrictive.