出乎我们意料的是，天气变得非常好。

(2)当定语从句放在主句后面时，as也并非永远等于which。

①当定语从句是否定句或表示否定时只能用which。

He came here very late，which was unexpected.

让人没想到的是，他来得非常晚。

②当as在从句中作主语时，后面常接动词的被动语态，如：be known，be said，be reported，be announced等。如果从句中的动词是主动语态，一般用which作主语。

She has been absent again，as is expected.

她又缺席了，这在意料之中。

Tom has made great progress，which makes me very happy.汤姆取得了很大进步，这使我很高兴。

③as常用在as seems likely，as often happens，as was said earlier，as I remember，as I understand，as appears等结构中，意思是"正如......"。

Jack has won the first prize，as often happens.

像往常一样，杰克得了一等奖。

She has read widely in Romantic literature，as appears from her essay.她广泛涉猎了浪漫主义文学，这从她的文章中可以看出来。

④as仍然保持作连词时常用的含义，意思是"和......一样"。

David is very tall，as are my brothers.

戴维很高，我的兄弟们也一样高。

2．大多数关系代词和关系副词均可引导限制性和非限制性定语从句，但that一般不用来引导非限制性定语从句。

误：Wang Fang passed her exam，that surprised me a great deal.

正：Wang Fang passed her exam，which surprised me a great deal.

王芳通过了考试，这让我很吃惊。

3．关系代词的省略情况。不同的关系代词在限制性定语从句中作宾语时可以省略，而在非限制性定语从句中，所有的关系词均不可省略。

This is the pen (which/that) I bought yesterday.

这是我昨天买的钢笔。

The pen，which I bought yesterday，has been given to my friend.我昨天买的笔，现已送给我的朋友了。

4．"介词＋关系代词"引导的非限制性定语从句

"介词＋关系代词"既可引导限制性定语从句，也可引导非限制性定语从句。"介词＋which"有时还可以与关系副词where，when，why等互换；"介词＋whom/whose"在定语从句中没有与之对应的关系词可替代。