都是it be开头，为何结构不同？
2015-08-18余可佳曾恒
余可佳 曾恒
2014年高考英语江西卷中有这样一道题：
It was the middle of the night my father woke me up and told me to watch the football game.
A. that B. as
C. which D. when
解析 D。此题考查it起头的句式。主句的主语it是人称代词，表示时间。when引导时间状语从句。句意是：当父亲叫醒我让我看足球比赛时，时间已经是半夜了。
英语中，以it be开头的句式繁多，而且结构相似，这一句式是高考的常考点。因此，悉心辨别这类句式有助于厘清句子结构，提高解题能力。
1 “it was/will be+某一时间点+when...”&“it was+时间点+定语从句”
“It was/will be+时间点”是一个主系表句式。其中，it是人称代词，表示时间。例如：
It will be evening when we get there. 我们到达那里时，将是傍晚了。
但是“It was+时间点”的句式后再接定语从句，就与强调句式非常相似。试比较：
It was seven oclock when I got up this morning. 我今天早晨起床时已是7点了。（it是人称代词，指代时间。表语seven oclock是对主语it的解释，后面的when引导定语从句）
It was at seven oclock that I got up this morning. 就是在今天早晨7点，我起床了。（at seven oclock是一个状语，与it没有指代或解释的关系，只是夹在It was...that中间，是被强调的部分）
2 “it is序数词that...” & it is time that...
“it is+序数词+that...”句型中的that从句的谓语用完成时态。如果主句中的be是is，从句谓语用现在完成时，主句中的be是was，从句谓语用过去完成时。如：
It/This the first time I been here. 这是我第一次来这里。
这一句型中，主句的主语it有时也可用this或that替代。例如：
This is the third cigarette that you have smoked this morning. 这是你今天早晨吸的第三支烟了。
然而，it is （high） time that...后接虚拟语气的从句，从句中的谓语多用过去式，有时也用“should+动词原形”，should不能省略，表示“早该……”。例如：
It is time that children should go to bed.=It is time that children went to bed. 孩子们该去睡觉了。
另外，“it is time+不定式”也能表示“该是做……的时候了”。例如：
When I was taken off the school football team because I was unfit， I knew it was time to quit smoking. 当我因为不健康被校足球队除名时，我知道该是我戒烟的时候了。
当这两个句式的主句谓语时态相同时，其结构非常相似。试比较：
It is the first time that we have seen a film in the cinema together as a family. 这是我们一家人第一次一起在电影院看电影。（陈述做某事的次数）
It is time that we saw a film in the cinema together as a family. 我们一家人早该一起在电影院看一场电影。（提出一条建议）
3 如果被强调的句式中带有定语从句、状语从句或名词从句，由于结构不同，构成强调句的引导词that的位置也有不同。试比较：
It is years of hard work that has made him what he is today. 是多年的辛苦劳作使他成今天的样子。（that构成强调句；what引导从句作宾语补足语）
It was then that Claire realized that Tony had opened the curtains of the front window. 就是在那时克莱尔才意识托尼已打开了前窗的帘子。（第一个that构成强调句；第二个that引导宾语从句）
It was when Asimov was eleven years old that his talent for writing became obvious. 在11岁时，阿西莫夫的写作才能才显现出来。（when引导时间状语从句；that构成强调句）
The patent was given in 1876， but it was not until five days later that Bell sent his first telephone message to his assistant Watson. 这项专利是1876年发布的，但是贝尔是在五天以后才跟他的助手华生通了第一次电话。（until引导时间状语从句；that构成强调句）
4 “it is+一段时间+since...”&“it was/will be+时间+until/till...”
“it is+一段时间+since...”是“it has been+一段时间+since...”口语化的结果，表示“自从……以来有……时间了”。例如：
It is just a week since we arrived here. 我们来到这里刚好一个星期。
It had been fifteen years since she left. 她离家15年了。
如果主句的谓语用一般过去时，从句谓语要用过去完成时。例如：
It was years since I had seen her. 我已经有好多年没见到她了。
since从句中的谓语是持续性动词时，计算时间要从这个动作的终点算起。例如：
It is ten years since we lived here. 我们没在这儿住已经有10年了。
然而，“it was/will be+时间+until/till/before...”表示到某事发生时要等多长时间。如果主句谓语是一般将来时，until， till或before从句的谓语多用一般现在时表示将来。例如：
It was only five minutes until her husband came back from work. 离她丈夫下班只有五分钟了。
It will be ten days until my birthday comes. 到我的生日还有十天。
要注意这两个句型的主、从句谓语时态的不同。试比较：
How long is it since China sent the first manned spaceship into space？ 中国第一次发射载人飞船进入太空到现在有多久了？
How long will it be until China sends the first manned spaceship into space？ 还要多久中国就会发射第一艘载人飞船进入太空呢？
5 “it is+形容词+主语从句”的句式中，形容词的意义变化会导致主语从句引导词的变化。如果作表语的形容词含有否定意义，主语从句常用whether引导。如果表语有肯定意义，主语从句用that引导。试比较：
It is uncertain whether the medicine will bring about side effect， although about two thousand patients have taken it. 尽管大约有2000个病人已经服用过这种药物，仍不确定这种药物是否会带来副作用。
It is certain that the medicine will bring about side effect. 无疑药物会带来副作用。
6 “it is+表语+主语从句”的句式变成感叹句时，如果表语是形容词，要用how引起感叹句；如果表语是名词，要用what引起感叹句。试比较：
How amazing it is that astronauts are exploring outer space！ 宇航员在外层空间探险真是惊心动魄！（amazing是形容词，所以用how引起感叹句）
What fun it is that we jump into a river to swim on a hot summer day！ 在夏天很热的日子跳河里游泳真有趣！（fun是名词，所以用what引起感叹句）
7 在“it is+名词+定语从句”的句式中，it是人称代词。如果定语从句的引导词在从句中充当主语或宾语，定语从句用that或which引导。如果定语从句的引导词在从句中充当状语，表示时间的定语从句用when或on/at which引导，表示地点的定语从句用where或in/on/at which引导，表示原因的定语从句用why或for which引导。试比较：
It is the most instructive lecture that I have attended since I came to this school. 它是我到这所学校以来听过的最有教育意义的演讲。
It is a job where you are doing something serious but interesting. 你正在做的是一项严肃而有趣的工作。
