简单地说：对过去情况作推测,用must have been；所对现在情况作推测,用must be.
比较：
He must be ill. 他一定是病了.（指现在有病）
He must have been ill. 他当时一定是有病 / 他一定生过病.（指过去有病）
He must be very rich. 他一定非常富有.（指现在有钱）
He must have been very rich. 他当时一定非常富有 / 他一定曾经富有过.（指过去富有过）
In order to be irreplaceable one must always be different
标签：must
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have to和must的区别
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情态动词must的用法
Must I, Miss Sophie
You must love me
There will be和there will have的区别
have been done和have done的区别
i must be you意思 是什么 大概的也行
have been to和have been有什么区别
Passwor总白难ds must match是什么意思
