Me, being polite as my mother told me - oh, thank you for your invitation but I’m sorry I can’t go. I have a meeting with the professor. I’m very sorry.
German – why are you apologizing? It’s not your fault. You don’t have to be sorry.
Me, trying to explain portuguese politeness – I know that. I’m not exactly sorry, it’s just a matter of speaking. Being polite, understand?
German – Hmmm...