must have done：表示对过去的肯定的逻辑推测,推定某件事情、动作或状态可能在过去发生过（存在过）.
He must have been a doctor,for he knows medicine so well.（他过去肯定是一位医生,他对医学如此了解.）
Somebody must have stolen the ring.（肯定有人偷了这枚戒指.）[该句所推断的偷窃动作发生在过去.]
The temperature must have dropped to below zero last night ,because the water is frozen all over.（昨夜的温度肯定降到零度以下,因为水都冻住了.）
如果'must + have + 过去分词'句型与by now连用,还可以表示对现在完成的动作和状态的肯定推测,但实质上还是指所推测的过去的动作,例如：
They started early this morning; they must have arrived by now.（他们今晨很早就出发了,现在肯定已经到了.）
中国学生的常见病句是：They…; they certainly have arrived.病句的句义是“他们当然已经到达了.”（这不是对过去的肯定推测,而是断定动作已经完成.）