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!
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2016-06-05T10:29:52.643
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grammar
Correct usage of 'you'?
Just to be sure, sarcasm does not work well with the Japanese language.

If you are trying to be Patronizing , I suggest taking the direct approach.

Patronizing someone

> おまえは使{つか}い物{もの}にならん

If you really want to use 「有用{ようよう}」 Complimenting a friend (named Joe)

> ジョー君{くん}は有用{ゆうよう}な人材{じんざい}だ

You could also use this,which has a slightly different meaning.

> ジョー君{くん}は優秀{ゆうしゅう}な人材{じんざい}だ

Complimenting a teacher,

> ありがとうございました。

I can't see a student ever saying that a teacher is needed in this way in
Japanese, so if you ever want to thank a teacher for being using just say
"Thank you"
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Patronizing saying of "you" is お前.

If you know their name, it is ～君、～ちゃん and only name for friends. ～さん, ～様 for
higher ranking persons.

If you don't know their name, we usually omit the subject "you" and ask their
names but if you know their positions like 部長, you say their position.

In addition 貴殿 and 貴台 are polite sayings of you but they are very literary and
we hardly say them now.
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