A fox may grow gray, but never good.

狐貍有可能會變灰，但絕不會變好。

gray 灰色的 grow gray 變灰

A leopard can’t change its spots.

豹子不能去掉它身上的斑點。

leopard 豹子，美洲豹 spot 斑點

You cannot make a crab walk straight.

你不能讓螃蟹豎著走路。

crab 螃蟹

old habits die hard

積習難改

He’s taken up smoking again. You know he had quit for about a year, but I guess old habits die hard.

他又開始抽煙了，之前他已經戒煙大約一年多了，可能還是積習難改吧。