Dear Mr. Wang,
I am very sorry that I shall be unable to come to your classes today owing to a bad cold. I enclose a certificate from the doctor who is attending me. He said it will be two or three days before I shall be able to resume my school.
David
April 20
王老师：
很抱歉，我因患重感冒今天不能到校。现附上医生证明，他说我两三天后才能到校学习。
大卫
四月二十日