teach your grandmother to suck eggs
US /tiːtʃ jʊər ˈɡrændˌmʌðər tə sʌk ɛɡz/
UK /tiːtʃ jʊər ˈɡrændˌmʌðər tə sʌk ɛɡz/

1.
insegnare a tua nonna a succhiare le uova, insegnare a un esperto
to try to teach someone something they already know, especially someone who is more experienced than you
:
•
I tried to explain how to use the new software to my boss, but it was like teaching my grandmother to suck eggs; she's been using similar programs for decades.
Ho provato a spiegare al mio capo come usare il nuovo software, ma era come insegnare a mia nonna a succhiare le uova; lei usa programmi simili da decenni.
•
He started giving me advice on cooking, but I've been a chef for twenty years. It was definitely a case of teaching your grandmother to suck eggs.
Ha iniziato a darmi consigli sulla cucina, ma sono uno chef da vent'anni. Era decisamente un caso di insegnare a tua nonna a succhiare le uova.