Give her the letter to read, then she'll understand.

      这里的 then为副词，用于祈使句后面一个句子的开头，含义是“若那样”，前面的祈使句相当于条件状语从句。

     Give her the letter to read, then she'll understand. = Give her the letter to read, and she'll understand. = If you give her the letter to read, she'll understand.