Relative Clauses

Do you know the difference between these two sentences?

My brother, who lives in Nottingham, is a doctor.
My brother who lives in Nottingham is a doctor.

Here's a clue! How many brothers do I have?
​Think about your answer then click "read more" to check.

​In the first sentence, I have one brother and he is a doctor, and by the way, he lives in Nottingham. I could say, "My brother is a doctor" and this is a complete sentence which you would fully understand. "Who lives in Nottingham" is extra information which is not necessary. The commas act like brackets. I could write:
My brother (who lives in Nottingham) is a doctor.

In the second sentence, I have more than one brother. Imagine I have two brothers and you already know this. You also know that one of them lives in Nottingham and the other lives in London. If I said, "My brother is a doctor," you would probably ask, "Which brother?" and I would tell you, "The one who lives in Nottingham."

My brother who lives in Nottingham is a doctor but my brother who lives in London is a lawyer.

This means the Nottingham brother is a doctor and the London brother is a lawyer. "Who lives in Nottingham" is necessary information because it explains which brother I'm talking about. It can't be taken out of the sentence without losing something important.

Let me give you some more examples:

Warsaw, which is the capital of Poland, has a population of 1.7 million.
"Which is the capital of Poland" can be taken out and you will still understand the sentence. There is only one Warsaw in the world so I don't need to explain which one. There are commas.

Can I borrow the book which you told me about last week?
"Which you told me about last week" explains which book I'm talking about and is therefore necessary information. "Can I borrow the book?" is a bit strange and incomplete. Which book? There's no comma.
 
One more thing - is it possible to replace "who" or "which" with "that"? In sentences without commas, it's possible to use "that". You can't use "that" in sentences with commas. 
​
My brother that lives in Nottingham is a doctor.
Can I borrow the book that you told me about last week?

By the way, this is British English. There is a small difference in American English. I think they always use "that" when there's no comma whereas in British English, you can choose.

Summary:
1. Commas act like brackets and are used when the information is not necessary.
2. When the information is necessary for full understanding, there are no commas.
3. Only use "that" in sentences without commas.


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