Dear Sam,
I was so glad to get your letter. Thanks a lot for writing me often.
You wrote that you had failed a maths exam and asked me what you should do. Well, if I were you, I would tell my parents the truth. I think your parents will understand and support you. Tell them you will do your best and study hard to retake the exam.
As for me I have never failed an exam. This is how I usually prepare: I study about two hours a day looking over my notes and practising a lot.
Well, I have to go now.
I'm looking forward to hearing from you soon.
Best wishes,
твое имя
Not употребляется во всех остальных временах, кроме Present Simple и Past Simple (используется didn't), а также если отрицание частичное, какого-то конкретного слова: Not all the people read this. Используются с модальными глаголами и to be: My cat is not white, I cannot swim.