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A Japanese person that was born in Nagoya wrote on instagram a very short sentence: 年賀状に使えるな that I'm not able to translate. Different online translators give opposite results. The TanoshiJapanese dictionary doesn't help me distinguish between meanings 1) and 3).
To break it down:
年賀状 "new years card"
に "as, for, for the purpose of"
使える "can use" "be usable" -- potential form of 使う
な -- sentence ending particle (終助詞) for 詠嘆, 推量, 納得, etc.
年賀状に使えるな。
The な here can indicate light exclamation, conjecture, or maybe conviction. So it can mean something like:
"It can be used as a new years card(, I think/suppose)."
"(I think / I'm sure) I can use it as a new years card."
A few examples of this な:
かわいいな。 Oh that's cute.
いい天気だな。 What a nice day.
お[腹]{なか}が[空]{す}いたな。 I feel hungry.
明日は晴れるな。 (I think) it'll be sunny tomorrow.
It cannot be negative imperative ("Don't ~~!") in your example since 使える is the potential form.
Compare:
使いな。 -- light command
使うな。 -- negative imperative
