Dear ，
I would like to know if I could ask for a casual leave of absence from your Oral English class this Wednesday morning.
Yesterday evening I got a phone call from my cousin, who is now running a small firm in suburbs of this city . He needs an interpreter for two days but cannot find the most reliable person， so he wants me to have a try. Personally ， I regard this as 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 get over the difficulty， I am confident that I ll do a good job and both of us will grateful.
Earnestly yours，
XXX
