请谈谈对这篇英语短文的理解!A man heard that a company needed an office wo

请谈谈对这篇英语短文的理解!
A man heard that a company needed an office worker.So he wrote and asked for the job.While he was waiting for an answer,a friend of his helped him to meet the head of the company,and then he got the job at once.Several months later,while the man was working in the company,he got a letter.It was sent from his old address.This letter said:
Dear Sir:
We are sorry to have to tell you that we cannot let you work in our company because we don't think that you can do the job well.
After he read this letter,the man laughed.But when he looked at the letter carefully,he saw he had singed it himself.
不是翻译,是谈谈理解.比如你认为他的工作会是什么?那封信又是怎么回事?为什么会已经有了他的签名?最后一段的it又指什么?