如果我寫信(我的朋友英文...)

hi, there are 3 ways to present ... you can say ...
1. Your English is improved a lot. (that means ... ur friend's English is still improving during this time)
2. Your English is better than before. (that means ... ur friend's English improved)
3. Your English is better. (that means .. ur friend's English is better when u tell him)

我會覺得改善左o既係英文所以subject應該係your english而唔係you.
Your english is improved會好過your english is improving 因為passive voice 暗示左係人為令到英文進步, 間接嘉許左友人

您好，我推薦您一個非常不錯的英語學習網站，里面有很多非常不錯的學習資源，您能從中得到不少幫助：

http://www.hkenglishstudy.info

祝您好運!

Your English has progressed a lot!

你可以給你朋友一點鼓勵
直接說
Your English is a lot better!!
或　Your English is improving!!

這是真心讚許
當然不用passive voice了