it is not difficult to find books about animals in our school library为什么用it,不用this,形式主语的辨别

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it is not difficult to find books about animals in our school library
为什么用it,不用this,形式主语的辨别

意思上
this 是:这个
that 是:那个
it 是:它
用法上
this that it 都可以直接作主语
也都可以做代词
指代事物
例子:this /that/it is an apple.
但是,只有it可以做形式主语
例:it is better to say hello to him.
it 是形式主语,真正的主语是to say hello to him.
it作为形式主语的时候可以代替动词不定式/ing/一个句子.
“this”、“that”既可以作形容词,又可以作指示代词;而“it”只能作代词.
指“物”
1.“this”指近物,“that”指远物,“it”没有远近之分.如:
This is a computer.这是一台电脑.(在近处)
That is a bike.那是一辆自行车.(在远处)
2.在回答“this”和“that”作主语的疑问句时,常用“it”代替“this”和“that”.如:
-Is this/that a car?这/那是一辆小汽车吗?
-Yes,it is./No,it isn′t.是的,它是./不,它不是.
-What′s this/that?这/那是什么?
-It′s a ruler.这/那是一把尺子.
3.陈述在一起的两样东西时,应先说“this”,后说“that”.如:
This is a book.That is a pen.这是一本书,那是一枝钢笔.