什么情况下肯定推测用it must be 什么情况下用he/she must be为什么推测人时 这几个人称代词都可用?

Question

什么情况下肯定推测用it must be 什么情况下用he/she must be
为什么推测人时 这几个人称代词都可用?

Answer 1

Most of times,it does not matter.When dealing with subjects emphasizing events or things,"it must be" would be better.When dealing with persons,"he/she must be" could be the best.
It must be true that hybrids use less gas because most people are buying the hybrids when the gas price going up.
He must be right that his wife gave birth to a boy because he was there when his wife delivered the baby.
It must be true when he said he got a son because he was there when his wife gave a birth to his first baby.