2. as引导的非限定性定语从句既可以放在主句之前，也可以放在主句之后，但which或who引导的非限定性定语从句不能放在主句之前。如：

As we all know, the moon is a satellite of the earth.

The large area is covered with thick snow, which affects people's life greatly.

3. 在限定性定语从句中，关系代词作宾语时可以省略；但在非限定性定语从句中，关系代词作宾语时不能省略。如：

The film (which) I saw last night is about a young teacher. (which可以省去)

She introduced me to her husband, whom I hadn’t met before. (whom不能省去)