我给你一篇例文吧
Dear teacher:
I really want to say thank to you because I have changed so much just because one sentence you said. I used to be a naughty boy/girl and not study hard. I thought you would punish me for my poor score, while you didn't. I was so touched by what you said that day and since then I changed my attitude, all because of your confidence with me and your encouragement to me! Thank you!