Complex sentence: Teach fish how to swim is an idiomatic expression dervied from the Latin proverb "piscem natare docem" which describes the self-sufficient perception of those who know how to do every thing better than the experts.
Split into simpler sentences: Teach fish how to swim is an idiomatic expression dervied from the Latin proverb "piscem natare docem". The phrase focuses attention on the self-sufficient perception of those who know how to do every thing better than the experts.