I didn't understand
I didn't quite understand.
Eu não entendi uma só palavra.
I didn't understand a single word.
Eu não entendi como de repente eu me tornei tão forte.
I don’t understand how I suddenly became so strong.
— Dangal (2016)
Eu não entendi o que ele disse.
I didn't understand what he said.
I didn't understand the joke.
At first, I didn't understand.
I didn’t understand that joke.
Eu não entendi completamente o ponto da sua pergunta.
I didn't fully understand the point of your question.