回答如下：by the time意为“到……时候（为止）”，是一个介词短语，在句中经常起连词作用，引导时间状语从句。用法归纳以下3种情况：

【1】如果从句中的谓语动词是一般过去时，那么主句中的谓语动词通常用过去完成时，表示“截止到从句动作发生时，主句动作已经完成，即过去的过去”。

By the time he was fourteen years old，Einstein had learned advanced mathematics all by himself. 到14岁的时候，爱因斯坦就已经自学完了高等数学。

By the time he returned home，the rain had stopped. 等他回到家时，雨已经停了。

By the time I got to the station，the train had already gone. 

By the time l got home，they had already left.

By the time I received his letter, he had left Shanghai.

By the time he was 12, he had traveled to more than 30 countries to get his diseases cured.

【2】如果从句中的谓语动词为一般现在时/或现在完成时（表示将来），那么主句中的谓语动词常为将来完成时，表示“截止到将来某一时间为止，主句动作将已经完成”。

By the time John finishes his homework， his classmates shall have left to play football. 到约翰做完作业时，她的同学将已离开去伦踢足球了。

By the time you get back，I shall have finished the work. 到你回来时，我将已经把活做完了。

By the time I get home，they will have left.

By the time I graduate next year, I will have lived here for 5 years. 到明年毕业时，我就在这里呆了五年了。

By the time the Olympic Games start, we will have finished/ finish the construction. 我收到他信时，他已经离开上海了.

【3】如果主句动作不强调已经完成，只是说明某个时候的状况，（主句是be的系表形式或者是表示像know, find, believe等表示认知的持续性动词，则往往用一般时态，不用完成时态。）此时主句不用完成时，而应根据实际需要选用适当的时态，此时by the time相当于when。

He was out of breath by the time he reached the top. 登上顶端时，他气喘吁吁。

By the time the bus came，so many people were waiting that it was a free fight to get on it. 公共汽车来的时候，等车的人很多，成了一场挤车的混战。

By the time he went abroad, he was fourteen. 出国那年他14岁。

By the time the war was over , death and suffering were to be seen everywhere. 在战争结束的时候，到处可见死亡和饥饿。 

By the time we arrived the meeting was over. 我们到达时，会议已结束了。

The injured man was fairly far gone by the time the ambulance arrived. 救护车到时那伤者已生命垂危。

By the time he is ten, he has outgrown his older brother. 他十岁的时候就已经长得比他哥哥高了。