“teach fish to swim”直接翻译成“教鱼去游泳”，引申为班门弄斧。

【例句】
He's an old dog in this line. Don't teach fish to swim.
他是这行的老手了，切勿班门弄斧。

You want to teach him english? You're just offering to teach fish to swim.
你想给他教英语？你这是班门弄斧。