The guests will have arrived.
这个句子究竟是什么意思？
这是个歧义句。它能很好地表明同一个语法形式 will have done 却具有两种完全不同的语法功能。
A. will 用作助动词,与 have done 连用,构成将来完成时:
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By next week, they will have completed their contract. 到下周他们就执行完合同了。
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By this time next year George will have taken his university degree. (GPU) 明年的这个时候,乔治将会拿到大学学位。
按照这种理解,你提问的句子的意思是:
客人们到时候就将到达那里。(The guests will actually be there by that time.)
B. will 用作情态动词, 表示对现在情况的“预测”“猜测”意思, 相当于 must 或“It is likely that...”:
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That'll be the postman. (CGEL) 可能是邮差来了。
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This will be the house you're looking for. (Palmer) 这想必是你在找的房子。
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She will have had her dinner by now. (CGEL) 现在想必她已经吃完饭了。
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You'll have heard of him? (Greene) 你可能听说过他?
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Sorry to be so late. You will have been waiting for some time. (Clark) 真对不起,我来得太晚了。你可能等了一些时间了。
（注意: will + 完成体不定式, 在这里仍解释为对现在状况的“推测”或“猜测”, 即对动作已完成的现在情况的“推测”或“猜测”。）
按照这种解释,你提问的句子的意思是:
客人们现在想必已经到了。(The guests have perhaps arrived.)