*We use non-defining relative clauses to add extra information about someone or something.We don't need it to understand.
*We use the following relative pronouns(which,who.whose or whom) to introduce a non-defining relative clause.
EX:John,who works with me,is very poor.We may say John is very poor.But who works with me is unnecessary information.
*That isn't used to introduce a non-defining relative clause.
EX:His car,which was very old,broke down after one kilo.
Which was very old is extra information.
*Compare the following sentences:
EX1:His brother,who works at the bank,is one of my friends.
EX2:His brother who works at the bank is one of my friends.
- Who works at the bank in the first sentence added extra information because he has only one brother who works at the bank.
-But who works at the bank in the second sentence is necessary because he has more than one brother.
EX:My friend ,whose car was damaged,is very rich.I have one friend so it isn't necessary.But If I have more than one friend I must say my friend whose car was damaged is very rich because it is necessary to define.
EX:The man, whom I know , is a doctor.
Whom I know isn't necessary because I know that man only.But If I know more than one man, it will be necesary so I must ommit the commas.