I didn't understand
Não entendi nada do que o professor falou.
I didn't understand anything the teacher said.
Amigo, não entendi nada do que você falou.
Friend, I didn't understand anything you said.
— Avatar: O Caminho da Água (2022)
I didn't understand the meaning.
Desculpe, porém devo confessar que não entendi bem seu comentário.
Sorry, but I must confess that I didn’t quite understand your comment.
Não entendi muito bem o que dizia.
I didn't quite understand what he was saying.
— Paris, Te Amo (2006)
I didn't understand what he said.
I didn't understand your explanation.
I pretended not to understand.