§52 仮主語、仮目的語の it
主語または動詞の目的語が「不定詞名詞用法、動名詞、名詞節」(= 真主語、真目的語)のとき。it = 仮主語、仮目的語
I know what it is to be poor.
What fun it was running around with those puppies!
It matters much to a man's character where he is born.
I have long had it on my mind to propose that we should part company.
He found it great fun teaching English to them.
He made it known that he had no intention to marry her.