write a letter的用法和样例：
- It happened/chanced that I was busy writing a letter when he came.
- Has she ever written a letter to Mark?
- I was writing a letter to my family.
- He is writing a letter now.
- While Mary was writing a letter, the children were playing outside.
- Is it necessary, do you think, to write a letter to the teacher?
- The duke wrote a letter with a quill pen.
- I was so annoyed that I felt impelled to write a letter to the paper.