I didn't understand
Je n'ai pas saisi le sens de la blague.
I didn't get the joke.
Je n'ai pas saisi ce que tu as dit.
I didn't understand what you said.
I didn't catch their name.
Je n'ai pas saisi l'intérêt de son discours.
I didn't understand the point of his speech.
Excusez-moi, je n'ai pas saisi.
Excuse me, I didn't understand.
Je n'ai pas saisi ton nom de famille.
I didn't catch your last name.
Je suis désolée, je n'ai pas saisi cela.
I'm sorry, I didn't understand that.
I didn't get it.