The letter served him as a bookmarker.

他将那封信用作书签。

youdao

Put a bookmarker into the book so that I can know where you have read.

书里面放上书签，我好知道你读到哪儿了。

youdao

Put a bookmarker into the book so that I can know where you have read.

书里面放上书签，我好知道你读到哪儿了。

youdao