2019屆高考英語語法定語從句要點專題精講
as he realized，i was very useful to him.
（在前）正像他所意識到的那樣，我對他非常有用。
air，as we know，is a gas.（在中）空氣，眾所周知，是一種氣體。
he was a foreigner，as i know from his accent.
(在后)他是個外國人，正像我從他的口音判斷的那樣。
he came late again，which made his boss angry.
（在后）他又來晚了，這一點使得老板很生氣。
在which引導的定語從句中，謂語動詞若是am，is，are，則這些系動詞不能省略，而as后面若是這種情況，則可以省略。如：
he is a teacher，as (is)clear from his manner.(is可以省略)他是個老師，這從他的言談舉止可看清楚。
he said he had never seen her before，which was not true.
（was 不可省略）他說他從來沒見過她，這一點不對。
當which引導的從句在意義上近乎并列關系時，可以用and this，and that代替，意思是“這件事”。如：
he changed his mind，which (and this，and that)made me very angry.
他改變了主意，這使我很生氣。
she has married again，which (and this，and that)was unexpected.
出乎意料的是，她又結婚了。
而as主要起與上下文連接的作用，表達說話人的看法、觀點，并指出主句內容的出處或根據(jù)等。如：
einstein，as we know，is a famous scientist.愛因斯坦，眾所周知，是位偉大的科學家。
as is announced in today’s newspaper，we must improve our style of work.
今天的報紙上說，我們必須改進工作作風。
作主語時，which既可以作系動詞be的主語，也可以作實義動詞的主語，而as只可以作系動詞be的主語。如:
he married her，which was natural.
（可用as代替which）他和她結婚，這是很自然的事。
he won a/the nobel prize in chemistry，which made him illustrious.
(不可用as代替which)他獲得了諾貝爾化學獎，這使他名揚天下。
當定語從句位于主句之后，純粹表示主句所述的內容，不帶有“如……那樣”的意思時，往往用which，而不用as。如：
they were invited to the state banquet，which was a great honor to them.
他們被邀請參加國宴，這對他們來說是莫大的榮幸。
we had to sleep in our wet clothes，which was most uncomfortable.
我們不得不穿著濕衣服睡覺，這簡直太不舒服了。
mummy always treats me just like a baby，which i can’t bear.
媽媽老把我當成小孩子對待，這讓我無法忍受。
當定語從句有“如同……那樣”的意思時，用as比用which更常見，而這些從句幾乎成為固定說法。如：
things will turn out contrary to one’s wishes，as is often the case.
事與愿違，這是常有的事。
as was natural， this inordinate hope was followed by an excessive depression.
這種過分希望之后，接著是極度的沮喪，是很自然的事。
chaucer is buried in “poet’s corner”，as might have been expected.
正如人們已經(jīng)預料的，喬叟被葬在“詩人角”。
the material is elastic，as is show in the figure.這種材料有彈性，如圖所示。