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Use commas to set off nonessential expressions--words, phrases, and clauses that are not necessary for the meaning or structure of the sentence. You can tell whether an expression is nonessential by trying to omit it. If the sentence makes sense and is structurally correct, the expression is nonessential and should be set off by commas.
Nonessential: Let's talk to Bob, who has experience in this area.
Essential: Let's talk to someone who has experience in this area.
My teacher confuse me.with this example.My brother who lives in RhonerPark is in jail.second phrase look the same the different is a(,)My brother,who lives in RhonerPark,is in jail.number one tell you i have more than one brother.this is correct?it look the same to me.