it's not easy to catch fish only with your hands.帮忙找一下it的用法

"it"在此处是作代词,代替后面的"to catch fish only with your hands",后面这个短语才是真正的主语.
一般因为"to do something"成分太长,放在前面会使句子有头重脚轻的感觉,读起来也不方便,所以久而久之,
it is adj. to do something差不多也就成了固定用法了.

it is +形容词+to do sth

做什么什么事 ...

it 是形式主语，真实主语是“catch fish only with your hands”这句话。正常语序是“Catch fish only with your hands is not easy”.