例如：This is the man (who gave me this book). 这就是给我这本书的人。
上述从句(who gave me this book)不能删去，如删去则成了This is the man.(这是一个人。)完全失去了整个句的意思，因此不能删去。
What is the name of the student who brought us the letter?
The students who find out about their dominant learning style can make their learning more efficient.
Is there anybody in this class whose father is a doctor?
The boy (who) is standing there is my brother.
I will give a present to those who come to class next Sunday.
The person whom you wrote a letter to has come to visit you.
This is the student whose father is a doctor.
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