by的用法，求助！

1）表示以一般的方法或方式。如：

No one in those days could live by writing poems.

（2）表示传达、传递的方式或煤介。如：

How did you send the letter, by airmail or by ordinary mail?

（3）表示用交通工具、通讯工具后接名词单数，不加冠词。如：

He came by train, but his wife came by bus.

注意下面两句的区别：

Did you come by train?

Did you come in his car / on my bike?

“by +抽象名词”构成的词组有：by accident / by chance / by diligence / by effort / by force / by heart / by luck / by mistake / by hard work.

将来完成时用来表示在将来某一时间以前已经完成或一直持续的动作。经常与before+将来时间或by+将来时间连用，也可与before或by the time引导的现在时的从句连用。

1) 构成will / be going to do sth.
2) 概念
a. 状态完成：表示某事继续到将来某一时为止一直有的状态。
b. 动作完成：表示将来某一时或另一个将来的动作之前，已经完成的动作或一获得的经验。
They will have been married for 20 years by then.
You will have reached Shanghai by this time tomorrow.
将来完成时的构成
将来完成时的构成是由"shall/will + have +过去分词"构成的。
Before long he will have forgo