One possible version:

Dear Jack,

I’ve received your e-mail and know that you are in bad mood now. However, I don’t think it’s necessary for you to be so disappointed for failing twice. You see, we will face many more exams in the future even if we leave school! So try the follow:

Collect the mistakes you made in the exams or in the exercises you did before, and analyse them.carefully. Try to avoid making them again in the next exam. Make good preparations and be confident.

As is known to us all, failure is the mother of success. Let every failure be your stepping stone. If you try your best and never give up, you are sure to succed in time.

Yours,

Jay

题目分析：本文是篇材料作文，材料非常简单，学生很容易把握写作要求和写作目的。考生在写作之前必须认真审题，首先要注意文章的时态，要求是你的好友Jack因两次考试成绩不理想而烦恼, 发来e-mail向你倾诉，你回复1.不必沮丧，认真分析(analyze)失败原因。 2．增强自信心，避免犯同样的错误。3．失败是成功之母，只要努力而不放弃，迟早会成功的。因此本文必须用第一、二人称适当变化。要用鼓励和建议的句式。注意不要根据材料逐字逐句地翻译，应该注意句子的衔接，以提高作文的档次。

【亮点说明】文中使用了非常好的短语和句子为文章增色不少，比如be confident，failure is the mother of success，whenever，in time，等等。并注意了句子的衔接如：So， However，You see，As is known to us all 等等。