形式主语it所引导的句子结构
当主语是动词不定式、动名词、主语从句时，往往把主语放在谓语动词之后，而用it 放在句首，作形式主语，放在谓语动词之后的主语是真实主语。例句：It is no use talking.
it引导的句子和定语从句有什么区别,比如 this is a famous movie,it is - ...
it 只是一个代词，可以在句子里作主语、宾语等，其本身并不是关系代词，因此也不“引导”从句。
你是问 it 作主语的的句子在做主句时和作定语从句时 有什么区别？—— 没什么区别，都是用作主语。需要明白的是，it 有时是作形式主语用，逻辑主语是后面跟着的不定式或者从句。例：
It is my job to help students learn English well.（to help students learn English well 是句子的真正主语，又称“逻辑主语”）
It was reported in today's news that some students would have missed Gaokao without taxi drivers' help.（that 从句是逻辑主语）
关于It的六种基本句型和例句
1. 陈述句的强调句，强调陈述句中的某个成分。
为了强调句子中的有些成分，要用强调结构It is（was）+被强调部分+that（who）+其他部分。被强调的成分可以是主语、宾语、状语，但不能是定语表语和谓语。这种类型的强调句主要注意一下几个方面：
1）It在这一结构中作主句的主语，所以即使被强调部分是复数名词，也要用is或was.
e.g. Tom and Mark often come to see me.
It is Tom and Mark Who often come to see me. （强调主语）
2）如果被强调部分是时间、地点、方式或原因也不用when， where， how， why， 而用that；强调人时可用who，也可用that.
e.g.（1）They set out the day before yesterday.
It was the day before yesterday that they set out （强调时间状语）
（2）We put off the sports meet because of the rain.
It was because of the rain that we put off the sports meet（强调原因状语）
（3）It was because she was ill that they didn‘t ask her to do the job. （强调原因状语从句）
3）被强调部分如果是原句的主语，who或that后面谓语的人称和数应和原句的主语一致。
— Personally， I think it is the salesmanager， rather than the salesgirls， ____ to blame.
—I agree with you.
A. is B. that is C. are D. who are
2. 一般疑问句的强调句，只需把is（was）提前。
e.g. （1） Was it in 1939 that the Second World War broke out？
（2） Is it Professor Wang who teaches you English？
（3） —Was it two months ago _____ you luckily got an opportunity to spend your holiday in Canada？
— _____. John was the lucky dog……
A. when； Not I B. when； Not only C. that； Not all D. that； Not me
3.特殊疑问句的强调句，强调特殊疑问句中的疑问词。
e.g. （1） Where did you find the wallet？
Where was it that you found the wallet
（2） When shall we meet？
When is it that we shall meet？
从上面的例子中我们可以看出，特殊疑问句的强调句是由“特殊疑问词+was（is） it that +陈述句”构成的。但是当含有特殊疑问词的强调句型用在宾语从句中的时候，上述句型应改为‘特殊疑问词+it was（ is） that +陈述句“，即采用陈述句语序。
e.g. （1） — where was it ____ the road accident happened yesterday？
— In front of the market.
A. when B. that C. which D. how
（2） I really don‘t know _____ you returned the book to me.
A. when it was that B. when was it C. when was it that D. when it was
4. 有时可用It might be ……that ……，和It must have been……that……的句型表示强调。
e.g. （1）It might be his father that you are looking for.
（2）It must have been his brother that you met.
5. 强调not……until结构中由until引导的短语（或从句）表示时间状语的句型是：It is（was） not until……+that……+其他部分。
e.g. 1）He didn‘t marry until forty.
It was not until forty that he married.
（2）Mr. Yang did not leave the lab until he got everything ready for the experiment.
It was not until he had got everything ready for the experiment that Mr. Yang left the lab.
（3）_____ he realized it was too late to return home.
A. Not until dark B. It was until dark that C. It was not until dark that D. It was dark that注意：如果not……until放在句首时就要用部分倒装句式，它的语序和在not……until结构中由until引导的短语（或从句）表示时间状语的句型中的语序不同。
6. 强调词it与先行词it的区别。
强调词it与先行词it的区别可以通过恢复原句的方式来判断，即如果把it is……that……去掉后剩下的成分仍然能组成一个完整的句子，这就是强调句型，否则就不是。
It is there that accidents often happen.
→ Accidents often happen there.
以上就是强调句型，被强调成分是状语，把it is……that……去掉，还原成后来的非强调句。
It is clear that not all boys like football.
上面的句子如果去掉it is……that……后就成了Clear not all the boys like football.很显然这不是一个完整的句子，因此不是强调句型，而是由it作先行词引导的一个主语从句。
7. 其他的一些强调句型：
It is/was+时间段+since从句，It +时间段+before从句，It is/was+时间点+when从句。
1）It is/was + 时间 + since …，从……已多久了。在这个句型中，主句通常用一般现在时，since从句用过去时，表示一个时间点。
e.g.（1）It is five months since I arrived in New York. 我到纽约已五个月了。
（2）It is five months since I was in New York.我离开纽约已经五个月了。
注意：例句中five months是作为一个整体看待的，所以用it is.在这类句子中，终止性动词表示从动作发生时算起；如果是状态动词或持续性动词，则从动作或状态结束时算起。
2）It is + 时间+ before …。在这个句型和上面一个句型中，时间都是时间段，如years，five days，twenty minutes等。不过主句的时态多用一般将来时或一般过去时，从句的时态要根据主句的时态进行变化。Before前的主句如果是肯定的，翻译成“在……之后才……” 主句如果是否定的，翻译成“不久就……”
e.g.（1）It was long before he gave an answer.
过了好久他才答复我。
（2）It was not long before he worked out the difficult maths problem.
不一会儿他就把那道数学题解出来了。
（3）It will be years before he comes back.
过几年他才回来。
（4）It was five o‘clock A he came to see me.
It was at five o‘clock B he came to see me.
It is five years D I left school.
It was five years C he joined the army.
A. when B. that C. before D. since
3）It was + 时间 +when从句在上述句型中it指时间，when引导的是时间状语从句
e.g.（1）It was at midnight that I got back home yesterday.
（2）It was midnight when I got back home yesterday.
第一句是强调句型，被强调的时时间状语，因此用介词短语表达，而第二句是一般句型，时间以名词的方式表达，用来作表语。
二、谓语动词的强调
it is……that……结构不能强调谓语，如果需要强调谓语，用助动词do， did或does.如：
Do come here tomorrow.
He did write to you last week.
Mary does study hard now.
想做英语的练习题的话我建议你可以去我要模考网看看，上面的题型挺全的
It is only 引导的句子
这句话是可以的。这是一个强调句型，要是把它变成正常语序就是：we can live a better life only when we take measures
it is/was 引导的句型
1.It is / was＋被强调的部分＋that(who)+剩余的部分.如：It wasn’t until he came back that I went to bed.直到他回来我才睡觉。(一定要注意被强调句型谓语动词否定的转移，及形式)。
It was because he was ill that he didn’t come to school today.只因为他有病了今天没有来上学。（只能用because而不能用for, as 或since）
It is I who am a student. 我确实是个学生。（句中am不能用are来代替。）
2.It is high time (time/ about time) (that) 主语＋should do / did+其它。（注意从句中的谓语动词用的是虚拟语气）
It is high time that we should go / went home.我们该回家了。
3.It is / was said ( reported…)＋that+从句. 如：
It was said that he had read this novel.据说他读过这篇小说。＝He was said to have read this novel.
4.It is impossible / necessary/ strange…that clause.(从句中的谓语用should＋do / should have done，其形式是虚拟语气。)如：
It is strange that he should have failed in this exam.真奇怪，他这次考试没有及格。
5.It is + a pity/ a shame…that clause.(注意从句中的谓语动词用should do或should have done的形式，但should可以省略。)如：
He didn’t come back until the film ended. It was a pity that he should have missed this film. 他直到电影结束才回来。他没有看到这部电影真可惜。
6.It is suggested / ordered/ commanded /…that +clause.(从句的谓语动词用should do, 但should可以省略。)如：
It is suggested that the meeting should be put off.有人建议推迟会议。
7.It is/was＋表示地点的名词＋where+从句。（注意本句不是强调句型，而是以where引导的定语从句。）如：
It was this house where I was born.请比较：It was in this house that I was born. (后一句是强调句型。)
8.It is / was +表示时间的名词＋when+从句。（注意本句型也不是强调句型，而是以when引导的定语从句。）如：
It was 1999 when he came back from the United States. 请比较：It was in 1999 that he came back from the United States.
9.It is well-known that＋从句。如：
It is well-known that she is a learned woman.
10.It is +段时间＋since+主语＋did. 请比较：
It was +段时间＋since+主语＋had done. 如：
It is five years since he left here.他已经离开这儿五年了。
It was five years since he left here.（同上）
11.It is +形容词＋for+ sb.+ to do. 如：
It is impossible for me to finish this work before tomorrow.我明天之前完成此工作是不可能的。
It is +（心理品质方面的）形容词＋of + sb. + to do. = 主语+ be +形容词＋to do.(常用的形容词有：kind, stupid; foolish, good, wise等。)如：
It is kind of you to help me.＝You are kind to help me.你真好给我提供了帮助。
12.It is／was not until…that