[KyleGoetz]

Originally Posted by Sashimister

「そんなこと分からないよ！あすかちゃんも分からない でしょう。」 sounds pretty natural. To make it sound even more, I'd say でしょ instead of でしょう. That would go with よ better.

I don't think too many native speakers would consider your second sentence as being "polite", though. With ちゃん and 分かる, it sounds fairly informal to me. Had you said something like あすかさんもお分かりにならないでしょう。or あすかさんもご存じないでしょう。, then I would have called it polite (and you would have sounded weird ).

I always seem to have trouble discussing plain/polite/informal/formal with Japanese learners. (This means I have trouble almost every day!) I'm the kind to go facepalm everytime someone tries tell me です/ます is polite or formal because that's considered so boringly average here in Japan.

It's like even though my family have been speaking Japanese for 2,000 years but I sometimes have a first-year Japanese learner tell me what to do with my Japanese. Not talking about you, KyleGoetz, though.

I completely understand that ます is average. It's just that darn terminology. To draw a contrast with the dictionary/plain form (I realize "dictionary" is just "plain non-past" technically), we call ます form "polite." I guess we could call it "non-plain"

So I didn't mean to imply with ちゃん and such it is actually polite per se. What I was just concerned about was ending the first sentence with ない and the second with でしょ（う） instead of だろう. It's just that, in my head, ない and だろう are "plain" form, so they need to be used together when talking to the same person. I didn't know you could mix and match plain and ます form when talking to the same person!

Thanks for the lesson!