本英文小白要给多年不见的外教老师写封信……看看有没有语法错误之类的。。拜托了^_^ Dear A

其实语法错误几乎没有。建议把
开头部分的“you must still...”改成“I guess you may still remember EF，a xx girl”，委婉一点。
“The reason why I ...，so...”，so换成其他词，because、so不能连用。
还有结尾的“I am so happy to write you an email.”“an”改为“the”，是“这封信”。
还有，感觉你写得应该突出一下，你写信的原因，比如说“我突然发现你的卡片，这勾起了我什么什么美好的回忆。”
还有，可以问老师好嘛，你文章都没有提到。
整体不错！

Dear Amber,
Hi ! I am Helen, the student you have taught in China.(I think you must still remember EF right?)
I'm thirteen now.
The reason why I write this to you is because was cleaning my room yesterday and I found a card (or maybe I can call it a letter?)which you gave me many years ago ,so I decide to write something to you too . How lucky I am to be your student ! How lucky l am to meet you in this big world !
I am so happy to write you this email !If you receive it , please reply to me at once!（＾_＾）
Sincerely,
Helen