关于while doing=while he was doing的疑问

问题描述

老师您好！有这样一个句子：

Sam had lost his wallet while taking his savings to the post office.

此句中连词 while 后接现在分词 taking his savings to the post office.

课文注释中说道与 while he was taking his savings to the post office 时间状语作用一致。该如何理解 while taking his savings to the post office？

感谢，盼复！

老师答疑

黄老师：

在英语复合句中，当状语从句的主语与主句的主语相同时，从句可以省略主语，并把谓语动词改为非谓语动词形式，意思不变。换言之，以后如果你看到 while / when / if / although 等连词后接非谓语动词时，意味着该非谓语动词的逻辑主语就是主句的主语，可以改写成一个状语从句。同样地，以后你写英语句子时，如果状语从句的主语与主句主语相同时，连词后可以使用非谓语动词形式。

When doing homework, he often listens to music. 

= When he does his homework, he often listens to music.

黎老师：

    如果主句的主语与when(while)从句的主语是同一个（些）人或者事物，而且从句的谓语动词是“be + -ing”,就可以把从句的主语和be动词一起省略（不能只省略其中的一个）。如：Sam had lost his wallet while taking his savings to the post office.= Sam had lost his wallet while he was taking his savings to the post office