show off before an expert/teach a fish how to swim
班门弄斧
Lu Ban (鲁班) was supposed to be a consummate carpenter in ancient times.
古代有一个建筑和雕刻技术非常高超的人，名叫鲁班。
It is said that he once carved a wooden phoenix that was so lifelike that it actually flew in the sky for three days.
传说他曾用木头制作了一只五彩斑斓的凤凰，能够在空中飞翔三天不掉下来。
Thus it was considered the height of folly to show off one‘s skill with an axe in front of Lu Ban.
谁敢在鲁班门前卖弄使用斧子的技术，也就是说，想在大行家面前显示自的本领，这种太不谦虚的可笑行为，就叫做“班门弄斧”( 这句成语有时也用作自谦之词，表示自己不敢在行家面前卖弄自己的小本领)。
【成语活用】
I’m making a fool of myself trying to show off before an expert like you.
在你面前班门弄斧，太不好意思了。
“班门弄斧”的英文表达，常用“to teach a fish how to swim”，也就是说“去教一条鱼怎么游泳”，鱼本来就会游泳，怎么还用人来教呢?所以英语中要表示一个人“班门弄斧”做无用的行为的时候，就会用“to teach a fish how to swim”。举例来说：
My son， fresh out of college， keeps telling me how to run my business. But I‘ve been making good money for 30 years so he’s teaching a fish how to swim.
我的儿子大学刚毕业，可是他却不断地来告诉我应该怎麼做生意.我叁十年来生意做得很好，赚了不少钱.所以，他来告诉我怎麼做生意简直是班门弄斧。
