poučovat někoho o něčem, o čem on už ví více než já
You're teaching your grandmother to suck eggs, Ted. I've been playing this game since before you were born! (Cambridge International Dict. of Idioms)
He´s always telling the director how to run the business; that´s like teaching his garndmother to suck eggs. (Longman Dict. of English Id.)
Don´t try to teach your grandmother to suck eggs. (Longman Dict. of English Id.)
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