it 做形式主语和however 用法

不管however以何种形式出现，它都不外乎以下两种用法。

1. 用作副词
(1) 表示让步：意为“无论如何”“不管怎样”，用来修饰形容词或副词，
其词序为：however+形容词或副词+主语+谓语。

这样用的however其实具有连词的功能，用以引导让步状语从句。

如：
however much he eats, he never gets fat. 不管他吃多少，他永远吃不胖。

用于此用法时，请注意以下几点：
①此用法属however所有用法中最重要的考点，同学们务请引起高度重视。

②这样用的 however 与 no matter how 大致同义。

如：
People always want more, however [no matter how] rich they are. 人总是富了还想再富。

however [No matter how] hard I worked, she was never satisfied. 无论我多么努力地工作，她从来没满意过。

③有时从句谓语可用情态动词。

如：
Don’t laugh, however funny it may be. 无论多么有趣也不要笑。

I’ll try to finish it in time, however hard it may be. 无论多么难，我也要按时完成。

④“however＋形容词或副词＋主语＋谓语”有时可以有所省略。

如：
I refuse, however favorable the conditions. 不管条件如何有利，我都不干。

(conditions 后省去了are)
I’d rather have a room of my own, however small (it is), than share a room. 无论房间多么小，我宁愿一个人住一间，而不愿意与别人合住一个房间。

A grammar rule, however true (it is), is useless unless it can be understood. 一条语法规则，不管如何正确，除非能懂，否则毫无用处。

(2) 表示转折：尤其用于谈及一个既成事实时，表示转折，其意为“可是”“仍然”等。

可放在句首、句中或句末，通常用逗号与句子其他成分隔开。

如：
My father, however, did not agree. 但是，我父亲不同意。

My room is small; however, it’s comfortable. 我的房间很小，但却很舒服。

He said that it was so; he was mistaken, however. 他说情况如此，可是他错了。

注：however不能像 but(但是)那样直接连接两个句子(注意正句中的标点符号)。

如：
我们都已尽了最大的努力，不过我们还是输了。

.
正：We all tried our best, but we lost the game.
正：We all tried our best; however, we lost the game.
正：We all tried our best. however, we lost the game.
(3) 表示惊奇或强调：相当于how ever的用法，其意为“究竟怎样，到底以什么方式”。

如：however did you get here without a car? 没有汽车你究竟是怎样来的呢?
No matter +疑问词或者疑问词-ever 含义为“无论……都, 不管……都”。

他们引导让步状语从句，并且可以互换。

It作形式主语常见句型
动词不定式，动名词或主语从句作主语时，通常把它们放在谓语动词前，而用it作形式主语，放到句首，常见句型有：
1. It + be + 形容词+主语从句，如：
It is uncertain whether he can come to Jenny’s birthday party or not.
2. It + be + 名词词组 + 主语从句，这类名词有：a fact, a pity, a shame, an honour，
a question等，如：
It’s a pity that you missed the exciting football match.
It remains a question whether he will come or not.
3. It + be + 过去分词 + 主语从句，这类过去分词有：announced, believed, expected, hoped, decided, reported, said, shown等，如：
It is reported that 16 people were killed in the earthquake.
It is not decided who will give the operation to the patient.
4. It + 不及物动词(seem, appear, happen等) + 主语从句, 这类不及物动词有：appear, happen, seem等，如：
It seemed that he didn’t tell the truth.
It happened that I was out when he called.
5. It + be + 形容词 + for sb.）+ 动词不定式
这类句型常用形容词easy, difficult, hard, important, possible，wise等作表语，有时候为了强调不定式动作的执行者，常在不定式前加for sb，如：
It’s necessary for the young to master two foreign language.
It is unwise to give the children whatever they want.
这类形容词有：easy, difficult, hard, important, possible，wise
6. It + be + 形容词 + of sb. + 动词不定式，这类形容词常是表示心理品质,性格特征的形容词，如：kind, nice stupid, clever, foolish, polite, impolite, silly, selfish, considerate等。

某人和这些形容词可以构成主系表结构。

如：It’s very kind of you to help me with the work.
=You are kind to help me.
It seemed selfish of him not to share his dictionary with others.
=He seemed selfish not to share his dictionary with others.
7. It + be +名词词组 + 动词不定式，如：
It is not a good habit to stay up too late.
8. It + be + 名词或形容词 + 动名词，这类名词和形容词常常是：good, no good, no use, a waste of, useless, senseless等，如：
It’s a waste of time talking to her any more.
It is no use arguing about the matter with him.
9. It + take ( sb. )+ 时间（金钱）+ 动词不定式，如：
It took the workers almost three years to finish building the dam.
It will take a whole day to get to the top of the mountain on foot.
同位语从句是用来解释说明某一名词内容或实质的句子。

他所修饰的名词为同位词，在同位语从句中，that 只起连接作用，不作任何句子成分，that 无意义。

同位语从句的引导词不能省略.。