核心问题
（A）They must have arrived by now.
现在他们想必已经到达。（张道真：《实用英语语法》，1995年修订重印本，外研社，p.253）
(B) They ought to have arrived by now. 现在他们想必已经到达。
must have arrived 与 ought to have arrived 在表“推测”时,意思是不是相同?
词典解答
在上面的句子里，must have arrived 与 ought to have arrived 都是推测“(他们)已经到达”，但是说话人对其陈述的肯定性程度却有很大区别。must have arrived 表明说话人对自己所描述的事件确信无疑，也就是说，说话人完全肯定其说法的正确性。而 ought to have arrived 则表明说话人对自己所描述的事件仅仅是个猜测而已，也就是说，说话人并不能完全肯定其说法的正确性。在翻译时，应把它们之间的区别表达出来：将 must have done 译为：“一定”“必定”“肯定……”“必然”“准是……”；而将 ought to have done 译为“可能……”“或许……”“也许……”“想必……”“该……”：
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“The lights have gone out.” “A fuse must have blown.” (AED) “灯灭了。” “一定是保险丝烧断了。”
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If he had started at nine o'clock, he ought to have been in London by eleven o'clock. (Palm-er) 如果他是九点钟出发的话，那他十一点钟该到达伦敦了。