You should write a letter to your teacher to let him know how thankful you are.
Why don't you write a card to your teacher, so he knows how much you appreciate him.
ーYou should write a letter to your teacher to let him know how thankful you are.
「先生に感謝を伝えるため手紙を書いたら?」
to write a letter to ... で「…に手紙を書く」
to let him know how thankful you are で「あなたがどれだけ感謝しているかを彼に知らせる」
ーWhy don't you write a card to your teacher, so he knows how much you appreciate him.
「あなたがどれだけ先生に感謝しているからを知ってもらうのに、先生にカードを書いたら?」
to write a card to ... で「…にカードを書く」
to appreciate で「ありがたく思う・感謝する」
ご参考まで!