It was they that (who ) cleaned the bedroom yesterday. (昨天,是他们打扫的卧室)这句话强调的是主语他们,that可以用who替换。
It was in the street that I met his sister。(是在街上,我遇到了他的姐姐)
这句话强调的是状语,在街上。
It is said that he has come to hangzhou。(据说,他已经来到了杭州)
这句话It只是形式主语,真正的主语是It 后面引导的主语从句。