is used to denote a command from a superior entity.
It cannot be negated.
この質問に答えなさい。 (Please) answer the question.
もう寝なさい (Please) go to bed now.
ゆっくり話しなさい。 (Please) speak slowly.
Abbreviation
Is sometimes abbreviated to just 〜な
お前も悪いから謝りな。 You were wrong, so please apologise.
Should not be confused with な (command), which means the exact opposite!
このクッキーを食べな Please eat this cookie
このクッキーを食べるな。 Don't eat this cookie!