①.as引导的这种定语从句位置比较灵活, 可位于句首, 句中, 句尾; 但which引导的这种定语从句只能位于主句之后

②. as还带有正如, 正象之意, 而which不含此意
a. The earth is like a ball, as everyone knows.
b. As everyone knows, the earth is like a ball.
c. They said they were French, which wasn’t true.

常见问题
一.定语从句还原法: 试题中有很多有关定语从句的, 这类题往往需选择引导词部分. 做好这类题的关键首先是要有这样的观念: 定语从句应有完整而合理的句子成分. 基于这一点, 我们可以用还原法来检验定语从句是否正确, 即把定语从句还原成为一个简单句. 具体做法是:

①.第一步: 替换. 将关系代词直接替换成先行词, 如果是关系副词, 先替换成介词+关系代词, 再将关系代词替换成先行词
②.第二步: 调整, 即调整语序, 看能否构成一个合理的简单句
例如, 还原以下例句a中的定语从句的过程是: 先将that / which替换成先行词the park, 再调整语序, 就能得到一个合理的简单句: We visited the park last year. 再如, 还原以下例句b中的定语从句的过程是: 先将where替换成in which, 再将which替换成先行词the park, 再调整语序, 就得到一个合理的简单句: We once played for a whole day in the park.

a. Do you remember the park that / which we visited last year?
b. Do you remember the park where we once played for a whole day?
c. Do you still remember the days that / which we spent together ?
d. Do you still remember the days when we were together?

二.分隔定语从句: 定语从句一般紧跟在先行词之后, 但有时也被分隔开来, 在阅读时要注意根据定语从句所表达的意思来分辨先行词是什么
a. I was the only person in the office who was invited.
b. He was the only one of the students who passed the GRE

高考英语备考语法精要讲解一般过去时
　　1、过去已经完成的动作、结束的事情，或在过去重复发生的动作
　　When I was a child we always went to the seaside in August.
　　2、used to :表示过去的习惯，现在不存在
　　I used to smoke, but I don’t any more.
　　3、用在it’s time, would rather, wish后面的从句中
　　It’s time you went home.
　　I wish I had a better memory.
　　I’d rather you came tomorrow.

4、在从句中用来说明现在还存在着过去的情况
　　I’m sorry we left Paris. It was such a nice place.
　　5、could, might, would, should等过去的情态动词可用来指现在或者将来
　　Could you help me for a moment ?
　　I think it might rain soon.
　　Would you come this way, please ?
　　Alice should be here soon.
　　6、一般过去时与现在完成时在时间上的比较
　　①现在完成时所用的时间词: since, ever since, for, recently, lately, so far, yet, all this year, all my life, ever, never, already, before, just, in / during the last /past + 时间段, etc.
　　②一般过去时所用的时间词yesterday, last week, in 1999, when, years ago, then, just now, etc.

高考英语语法：谓语动词
总述: 谓语动词的变化形式取决于时态和语态: 英语的时态有十六种, 中学英语中有十二种常见的时态; 语态有两种, 即主动语态和被动语态
1.在主动语态中, 各种时态的谓语动词的一般形式如下:
一般进行完成完成进行
现在do / doesam / is / are doinghave / has donehave / has been doing
过去didwas / were doinghad donehad been doing
将来shall / will doshall / will be doingshall / will have doneX

如果谓语是be动词, 则可用be动词的适当形式( be / am / is / are / was / were / been)代替以上表格中的黑体字部分. 此外, 过去将来时的形式是: should / would do或should / would be
2.在被动语态中, 各种时态的谓语动词的一般形式如下:
一般进行完成完成进行
现在am / is / are doneam / is / are being donehave / has been doneX
过去was / were donewas / were being donehad been doneX
将来shall / will be doneXshall / will have been doneX
另外, 过去将来时的被动语态形式是should / would be done

②.表示客观事实或普遍真理
a. Japan is to the east of China.
b. The sun rises in the east.
c. A horse is a useful animal.

③.表示将来确定会发生的动作(如己安排好或计划好的动作或按时刻表将来一定会发生的动作), 可以这样使用的动词有: go, come, leave, start, arrive等
a. My train leaves at 6:30.
b. The football match is played the day after tomorrow.

④.在时间和条件状语从句中可用一般现在时动词代替一般将来时动词
a. I will let you know as soon as I hear from him.
b. We will go if it is fine tomorrow.
c. I will be away when he arrives.
d. We shall not begin the discussion until he arrives.
e. Mother, I won’t go out unless you agree.
f. Don’t try to run before you begin to walk.

⑤.在某些以here / there开头的句子中用一般现在时动词表示现在正在发生的动作
a. Here comes the bus.=The bus is coming.
b. There goes the bell.
⑥.在体育比赛过程中, 解说员叙述迅速, 短暂动作时, 可用一般现在时, 表示正在进行的或刚刚发生的动作
a. Ma Lianbao passes the ball to Mu Tiezhu, Mu shoots – a fine shot !
⑦.在剧本或图片的说明文字中, 用一般现在时表示动作
a. When the curtain rises, Juliet is sitting at her desk. The phone rings. She picks it up and listens quietly.

二.现在进行时:
1.构成: am / is / are doing
2.用法:
①.表示说话时正在进行着的动作, 或现阶段正在进行而说话时不一定正在进行的动作
a. I am writing a letter.
b. My mother is making a dress these few days.