teach fish to swim字面意思教鱼游泳，引申为班门弄斧。

【例句】
He is an old dog in this line, so don't teach fish to swim.
他是干这行的老手，就不要在他面前班门弄斧了。

Come on. What you are doing is to teach fish to swim.
行了，你这是关公门前耍大刀。