indefinite vs definite relative clauses.
Hi guys. Can anyone tell me the differences between indefinite and definite relative clause. When to use comma and which relative pronoun should we use ('which' or 'that') for things in both clauses?

Question

indefinite vs definite relative clauses.
Hi guys.  Can anyone tell me the differences between indefinite and definite relative clause. When to use comma and which relative pronoun should we use ('which' or 'that') for things in both clauses?

Answer

Of course. I think you mean defining and non defining clauses.
For example- My brother, who lives in London, is a dentist. (non defining, I have only one brother)
My brother who lives in London is a dentist (I have more than one brother- I am defining which brother I mean)
Maybe this link will help.
http://dictionary.cambridge.org/grammar/british-grammar/relative-clauses-defining-and-non-defining

Answer

Short answer:

If essential to the meaning of the sentence => no commas (use "that")

If nonessential to the meaning of the sentence => use commas (use "which")