it is quite interesting to climb hill

是to do,
是“不定式”做“主语”
前面it是形式主语
如果说成To climb hill is quite interesting ,意思也是对的,
外国人也能听懂,但是不够通俗而已.
就跟汉语的“今天见到你我很高兴”和‘我今天很高兴,因为我见到你了’两句话的意思差不多,语法都对,只是说的符不符合语言习惯而已.
还有,应该是it is quite interesting to climb the hill
或者it is quite interesting to climb hills.