Never offer to teach fish to swim.

不要班门弄斧。原意为：决不要教鱼游泳。

Never offer to teach fish to swim.

不要教鱼儿游泳。不宜译作“不要班门弄斧”。

Facing the teacher. I can't teach fish to swim.

在老师面前，我不能班门弄斧。

If you are not an expert, don't show off your skills as it says "never offer to teach fish to swim."

别班门弄斧 不要关公面前耍大刀 班门弄斧 不要教鱼游泳 字面意思：不要教鱼怎么游泳。通常意译为；

She's seasick? I'd sooner teach a fish how to swim.

她会晕船？那鱼也得学游泳了。