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
2. It is + adj. + of somebody to do something