pen friend

例句：

1. 1. I've been writing to my pen friend for months.
   我已经和我笔友写了几个月的信了。
2. 2. He decided to write a letter to his pen friend.
   他决定给他的笔友写信。
3. 3. They exchanged letters with their pen friends from different countries.
   他们与来自不同国家的笔友交换了信件。