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

此句中连词 while 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？

【答】while taking his savings to the post office， 属于while he was taking his savings to the post office的省略说法。

详情可参考：

http://www.yygrammar.com/Article/201501/3815.html