Defining relative clauses and Non-defining relative clauses(MĐQHĐ & MĐQHKĐ)

Defining relative clauses give essential information to define or identify the person or thing we are talking abou(MDQHXĐ cung cấp thông tin cần thiết để xác định hoặc nhận diện một người hay điều mà chúng ta đang nói đến)
Non-defining relative clauses are always separated from the rest of the sentence by commas.( Mệnh đề quan hệ không xác định là luôn luôn được ngăn cách ói MĐ c̣n lại của câu bằng dấu phẩy)

Ex1: Elephants who marry mice are very unusual.
Ex2: My friend John, who went to the same school as me, has just written a best-selling novel .
Ex3: Let's go to a country where the sun always shines.
Ex4: Elephants, which are large and grey, can sometimes be found in zoos

1. In non-defining clauses, you cannot use ‘that’ instead of who, whom or which.
2. You cannot leave out the relative pronoun, even when it is the object of the verb in the relative clause.
3. Non-defining clauses can be introduced by expressions like all of, many of + relative pronoun:
Examples: There were a lot of people at the party, many of whom I had known for years.