Please correct the mistakes.

"Take care of your studies, otherwise you'll regret it one day," my English teacher told me once. Yet, it was too late when I recognized what he really meant. I didn't know how important learning is (or was?). When problems got worse my teacher caught me again- his mood was extremely bad then- and said "if you don't improve your behaviour this time, you will be excluded from school." Unfortunately, he was right. And I was expelled from school. I know that crying for old days won't do me much for me; but I often find myself ungrateful to my English teacher. I feel sorry I didn't take his words seriously. Nevertheless, his words will always remain deeply carved into my memory.

Great job.

"Pay attention to your studies, otherwise you'll regret it one day," my English teacher said to me once. Yet by the time I understood what he really meant it was too late. At the time I did not understand just how important learning is. When the situation got even worse, my teacher rebuked me again. By this time, he was very angry and he said to me, "if you don't improve your behaviour this time, you will be expelled from school."  Unfortunately, he was right and I was indeed expelled. I know that crying over the past will not help me now, but I often find myself resentful towards my English teacher/ I often regret my ingratitude towards my English teacher. (not sure what you're trying to say)

I regret not taking his words seriously. Nevertheless, they will always remain ingrained in my memory.