有一读者问这样一道选择题：
(1)I have been here for four years; by next summer I _____.
A. would be graduated B. shall have graduated
答案为B，为什么不用A? 将A改为would have graduated可以吗？
这里的by意指“在……之前”，故而by next summer之后应用将来完成时。这种by常与完成时态连用，再如：
(2)By 2008 the number will have grown to 1，400 millions.
would have graduated可用，但须改变谓语的时态，如：
(3)He said that by next year he would have graduated from the university.
