在英语中 什么情况下用it造句 例如 it is rude to stare at others 为什么要用it啊

it [it]
[无人称代词] it[所有格] its [宾格] it; [复数] they,[所有格] their或theirs[宾格] them;
pron.
1.[指无生命的东西、动物、植物]它；这；那：
It is a table.
这是一张桌子.
Is this the peony?No,it is the tree-peony.
这是芍药吗?不,这是牡丹.
2.[指已提到或将提及的事物、活动、经验、抽象观念等]：
Although he didn't like it,I decided to see the movie anyway.
尽管他不喜欢这部电影,我还是决定去看一看.
I remembered he told me about it.
我记得他跟我提过这件事.
3.[在不分性别或情况不详时指]：
Who is it?It's me.
谁呀?是我.
4.[作无人称动词的主语,表示天气、时日、距离、状态、温度等等]：
It rains.
下雨了.
It is 5 o' clock in the afternoon.
现在是下午5点钟.
5.[作先行代词,引导作为逻辑主语或宾语的从句]：
It is certain that we shall succeed.
我们一定会成功的.
It is no use trying.
试也无用.
6.[作先代词,用以构成强调句型]：
It was here that I first met him.
这就是我与他初次见面的地方.
It is I that am fortunate.
这幸运的是我.
7.[口语中作某些动词或前置词的含糊的宾语,用以构成习语]：
to fight it out
战斗到底
to get with it
振作精神
8.[用在临时用作动词的词之后,用以构成习语]：
to lord it over others
对人称王称霸
9.[口语中作表语,指绝妙的人或物；理想的东西；登峰造极]：
In a lilac sun bonnet she was it.
她戴着一顶浅紫色遮阳帽,真是漂亮极了.
The steak was really it.
这牛排棒极了.
10.[加于主语名词后,起重复作用,常见于诗歌或非规范语]：
The horse it ran away.
那匹马呀,跑掉了.
11.[对幼儿用语,你]：
Did it hurt its little hand?
小乖乖手碰疼了没有?
n.
1.[口语]性感；性交
2.[美国俚语]傻瓜；笨蛋
3.(游戏“瞎子摸鱼”中的)摸人者；捉人者
4.[俚语]讨厌的人
5.[英语口语]意大利苦艾酒
6.[常作 I-]自大的人；自负者
adj.
[方言]它的
IT
abbr.
information technology 信息技术
It.
abbr.
1.Italian
2.Italic
3.Italy
it [it]
n.
the branch of engineering that deals with the use of computers and telecommunications to retrieve and store and transmit information
以上来源于:WordNet