what，where，which，who，whom，whether，How，that（与形式主语it配合使用）。
What he said is not important.
Where you live is important.
Which country you are going to does matter.
Who did it will make things different.
With whom you deal with is the key point.
Whether you speak English is quite important.
How others think is not the problem.
It is necessary that……
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