there is表示“有”的意思,例如：
There is apple on the desk.這兒有一個(gè)蘋(píng)果在桌上.
I.It is + adj.+ to do something結(jié)構(gòu)
　　句中it是形式主語(yǔ),真正主語(yǔ)是動(dòng)詞不定式to do something.因此也可以表達(dá)為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.
　　本句形式主語(yǔ)為it.真正主語(yǔ)to drive…grapevines移到了句子后部,使句子變得平穩(wěn),不致主語(yǔ)太長(zhǎng).如果把真正主語(yǔ)放在前面,那么整個(gè)句子就顯得頭重腳輕.因此,就用it作為先行主語(yǔ).
　　這種結(jié)構(gòu)還有兩種動(dòng)詞不定復(fù)合結(jié)構(gòu).
　　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.
　　此句真正的主語(yǔ)是動(dòng)詞不定復(fù)合結(jié)構(gòu)for them to fly.其中them是to fly的邏輯主語(yǔ).
　　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
　　答案為D.這里sensible跟邏輯主語(yǔ)you的關(guān)系是表達(dá)人的特點(diǎn)、性格、本質(zhì)等.像此類(lèi)形容詞還有kind,foolish,clever,wise,friendly等.
II.It is +名詞+ to do something結(jié)構(gòu)
　　e.g.It is great fun to see the pandas walking in the zoo.
III.另外,it作先行主語(yǔ)時(shí)還有其他用法
　　1.it +及物動(dòng)詞+賓語(yǔ)+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 +名詞/形容詞+ 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