It is + adj. + to do something結(jié)構(gòu)
　　句中it是形式主語,真正主語是動詞不定式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.
　　本句形式主語為it.真正主語to drive…grapevines移到了句子后部,使句子變得平穩(wěn),不致主語太長.如果把真正主語放在前面,那么整個句子就顯得頭重腳輕.因此,就用it作為先行主語.
　　這種結(jié)構(gòu)還有兩種動詞不定復(fù)合結(jié)構(gòu).
1. It is + adj. + for somebody to do something
2. It is + adj. + of somebody to do something