Dear Mr. Smith,
On account of an attack of the flu with a severe headache, I shall be unable to attend classes today, for which I submit a certificate from the doctor. I shall be much obliged if I can get your permission.
Respectfully yours,
John Warner
Dear Mr/Ms：
Today I'm writing to you to ask for a five-day leave, for I've got cold last night with carlessness.
This morning my mother took me to see the doctor,who told me to stay in bed for some days. So I am very sorry to be absent from school, especially your interesting lessons. I'll be sure to make up for the missed lessons after I recover from the illness.
Yours ever,
xxx.
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