【问】The guests will have arrived.
 这个句子究竟是什么意思?
 【答】 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：
 1. By next week，they will have completed their contract.
 到下周他们就执行完合同了。
 2. 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 likelythat...”：
 1. That'll be the postman. (CGEL)
 可能是邮差来了。
 2. This will be the house you're looking for. (Palmer)
 这想必是你在找的房子。
 3. She will have had her dinner by now. (CGEL)
 现在想必她已经吃完饭了。
 4. You'll have heard of him? (Greene)
 你可能听说过他?
 5. 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.)