Defining vs. Non-Defining Relative Clauses
Defining Relative Clauses (No Comma, Necessary Information)
These clauses give essential information about the noun.
The girl who sits next to me is my best friend.
The car that I bought last year is still new.
Non-Defining Relative Clauses (Comma, Extra Information)
These clauses add extra, non-essential information.
Use who or which, never that!
My brother, who lives in London, is coming to visit.
My house, which was built in 1920, is very old.