Relative Pronouns with identifying and non-identifying

[Yukicheng]

Hello teachers,

I'm a ESL student, and I always found the grammar is confusing like relative pronouns with identifying / non-identifying.
I don't understand when should I add "commas" before "which" and "who"
For example:
1. The dress,which was a family heirloom,was worn at the wedding.
2. The book which I just finished is by Eddie Hoffman.
Why I have a commas before which in question 1?
Why I don't have a commas in question 2?

3. My sister,who lives in New York City, comes to visit me in August.
4. I have three sisters. My sister who lives in New York City comes to visit me in August.
Question 3 is adding additional information about the noun.
How about question 4?

Thanks,
Yuki

 

[Matthew Wai]

In #2, there is more than one book, and you are referring to the one you just finished.
In #4, you have more than one sister, and you are referring to the one living in New York City.

http://grammar-monster.com/glossary/restrictive_clause.htm
http://grammar-monster.com/glossary/non-restrictive_clauses.htm
The above explanations about restrictive and non-restrictive relative clauses could be helpful to you.

 

[Matthew Wai]

1. I have read many books. The book which I just finished is by Eddie Hoffman.
2. I bought a book last month. The book, which I just finished, is by Eddie Hoffman.

Is that correct?

 

[Yukicheng]

Note my corrections to your original post, Yukicheng;

I don't understand when should I I should add "commas" (no inverted commas) before "which" and "who".

1. The dress,which was a family heirloom,was worn at the wedding. (space after commas)
2. The book which I just finished is by Eddie Hoffman.
Why do/must/should I have a comma[STRIKE]s[/STRIKE] before "which" in question 1?
Why I don't have a comma[STRIKE]s[/STRIKE] in question 2?
3. My sister,(space)who lives in New York City, comes to visit me in August.

Thanks Piscean!
Can you please explain to me why should I have a comma in question 1 and why I don't have a comma in question 2?
Thank you

 

[Matthew Wai]

In #1, the non-restrictive clause gives extra information about the dress and is not essential for understanding.
In #2, the restrictive clause identifies the book you are referring to and is essential for understanding.