A letter from a pen-friend的用法和样例：
- She was giggling at a letter from a boy.
- I have received a letter from a friend inviting me to work in a company in another town.
- I receive a letter from her in her neat script.
- Then he got out a letter from a bunch in his coat pocket.
- We are expecting a letter from her.
- Every now and then,she received a letter from a former student.