Question

我自己写的英语作文，麻烦帮改一下，清楚一点！

Answer 1

有一些用法不大合适，时态和三单也有些错误，但大体上不错，意思也清楚。我给你重写一下：
I heard that you were ill. I am very much concernd, so I write the letter to you and tell you something about keeping healthy.
Firstly, you should take some exercise to build up you body. It will benefit you forever. Secondly, you should pay more attention to your drink and food. Don't eat too much meat and drink coffee, because it is bad to you. So please eat more fruits and vegetables. If you want to drink, please drink boiled water.
Hope this can help you.
Best wishes.

追问

貌似应该是 build up your body?

追答

你是对的，我漏了字母r。谢谢！

Answer 2

Dear Jack,
I'm sorry to hear that you were ill. I write to you to give you some tips about staying healthy, because I'm concerned about your health. First, you should exercise often in order to have a strong body; working out benefits you forever anyways. Secondly, you should pay attention on what you are drinking and eating everyday; for instance, too much meat and coffee might be bad for you. Thus, add more fruits and greens into your diet is good. More importantly, water, water is what you should be drinking. Hope this helps!

Best wishes,

Yours
Lihua

I hope you learn from this, you have some small grammar issues.