【正确答案】Dear Smith,
I would like to know if I could ask for a casual leave of absence from your oral English class this Thursday morning.
Yesterday afternoon I got a phone call from my cousin, who is now operating a small company in suburbs of this city. He need an interpreter for two days but cannot find the most trustworthy person, so he wants me to make an attempt. It seems to me that this is a golden opportunity to put what I have learned in your class into practice, but I need your permission of absence. If you think I may go and help him overcome the difficulty, I am confident that I'll do a good job and both of us will be thankful.