8.y the time引导的时间状语从句

由by the time引导的时间状语从句要注意时态的变化:在一般情况下，如果从句的谓语动词用一般过去时，主句的谓语动词用过去完成时;如果从句的谓语动词用一般现在时，主句的谓语动词用将来完成时。

By the time you came back，I had finished this book．

到你回来时，我已经写完这本书了。

By the time he was fourteen years old，he had learned maths all by himself．(95人教版高二下第2页)

到他14岁的时候，爱因斯坦已经自学了数学。

By the time you come here tomorrow，I will have finished this work．

你明天来这儿的时候，我将已经完成此工作了。