there is表示“有”的意思，例如：
There is apple on the desk.这儿有一个苹果在桌上。
I. It is + adj. + to do something结构
句中it是
形式主语，真正主语是
动词不定式to do something。因此也可以表达为to do something + is +adj.。
e.g. It is amazing to drive past fields which seem to go on forever or whole hills covered with neat rows of grapevines。
本句形式主语为it。真正主语to drive…grapevines移到了句子后部，使句子变得平稳，不致主语太长。如果把真正主语放在前面，那么整个句子就显得头重脚轻。因此，就用it作为先行主语。
这种结构还有两种动词不定复合结构。
1. It is + adj. + for somebody to do something
e.g. It was very difficult for them to fly because men have not got wings.
此句真正的主语是动词不定复合结构for them to fly。其中them是to fly的逻辑主语。
2. It is + adj. + of somebody to do something
e.g. It is sensible_____ you to bring an umbrella in such weather.
A. in B. for
C. with D. of
答案为D。这里sensible跟逻辑主语you的关系是表达人的特点、性格、本质等。像此类形容词还有kind, foolish, clever, wise, friendly等。
II. It is +名词+ to do something结构
e.g. It is great fun to see the pandas walking in the zoo.
III.另外, it作先行主语时还有其他用法
1. it +
及物动词+宾语+不定式
e.g. It cost me 100 dollars to repair the car.
It takes me two hours to finish my homework every day.
It gave me great pleasure to watch them.
2. it + be +名词/形容词+ doing
e.g. It is no use studying for an exam at the last minute .
It is a waste of time talking to him.
It is pleasant sitting here