英语翻译我的数学老师；当我们第一次见他,看起来好苍老,头发白了好多.当他做自我介绍的时候,我们才知道他才40多.他有时严

My math teacher,
When we first met him, look so old, hair white a lot of. When he doing self-introduction, we didn't know he was just more than 40.
He sometimes strictly something interesting. We in his class, very happy. So we in his class more and more active