it做形式主语 必须用that引导从句吗

it做形式主语 必须用that引导从句吗?回答是：不是必须..
it做形式主语,后面内容做句子真正主语,除了由主语从句充当之外,还可以由不定时,不定时短语,动名词以及动名词短语充当,
例如下面句子
It is very useful to learn English so well
It is the police's duty to protect the peope's life and properties
It is not so effective learning English without practising speaking English with others as often as possible
It is his duty taking good care of all his children.
以上回到,

不是的。采用it做形式主语只是为了保持句子结构平衡而已，并不影响其语法功能，否则主语从句岂不千篇一律了吗？