it's hard to know the true nature of a man

it是形式主语,动词不定时短语to know the true nature of a ma才是真正的主语。所以,其实这句话就是：To know the true nature of a man is hard。很多时候,动词不定式做主语时,都放在句尾,这样可以避免句子头重脚轻。又如：It is very convenient to take a bus。有时还可以加上for或of引出的介词短语来引出逻辑主语。例如：It is very convenient for you to take a bus。It is very clever of you to kill her。如果不懂,请查阅语法书中,动词不定式部分。