以下两题请老师解答：

1、I____my letter by the time you come back.

A.have finished B.shall finish C.shall have finished D.shall be finishing(答案选C。请老师详细讲解一下C答案的时态在本题中的具体体现？）

     2、By next summer John____in his factory for three years.

A.have been working B.will have been working C.will be working D.has worked(答案选B。请老师详细讲解一下B答案的时态在本题中的具体体现？）

答案：

1、I____my letter by the time you come back.

A.have finished B.shall finish C.shall have finished D.shall be finishing(答案选C。请老师详细讲解一下C答案的时态在本题中的具体体现？）

在by the time 结构中主句多用完成时，come 用的是一般现在时，从句用将来完成时。 will/shall+不定式完成体表示将来某一时刻之前业已完成的动作。意思就是当你回来时，我将完成了这封信。

2、By next summer John____in his factory for three years.

A.have been working B.will have been working C.will be working D.has worked(答案选B。请老师详细讲解一下B答案的时态在本题中的具体体现？）

这句话的意思是到明年夏天为止，约翰工厂已工作了三年。will/shall+不定式完成进行体表示一个已经开始的动作到将来某一时间仍在进行。