I need a little help with the phrase 'You are...' For simplicity, I'm going to you the example: 'You are helpful.' It's an awkward example, yes, but it's the best I could find. :P
Are these correct?
Patronizing someone, being sarcastic: Anata wa yūyō desu.
Complimenting a friend (named Joe): Joe-kun wa(?) yūyō desu!
Complimenting a teacher: Sensei yūyō wa(?) desu!
However, I struggle with addressing someone whose name you don't know politely. You can't just say 'Yūyō desu' if they don't know you're addressing them, and 'anata wa' can sound rude!
Any help would be appreciated!