Using 「~なさい」 to make firm but polite requests
「なさい」 is a special honorific conjugation like 「する」. It is a soft yet firm way of issuing a command. It is used, for example, when a mother is scolding her child or when a teacher wants a delinquent student to pay attention. Unlike 「ください」, 「なさい」 only applies to positive verbs and uses the stem of the verb instead of the te-form. It also cannot be used by itself but must be attached to another verb.
Using 「なさい」 to make firm but polite requests
Conjugate the verb to its stem and attach 「なさい」
例) 食べる → 食べなさい
例) 飲む → 飲み → 飲みなさい
例) する → し → しなさい
Examples
(1) よく聞きなさい!- Listen well!
(2) ここに座りなさい。- Sit here.
You can also drop 「さい」 portion of the 「なさい」 to make a casual version of this grammar.
(1) まだいっぱいあるから、たくさん食べな。- There's still a lot, so eat a lot.
(2) それでいいと思うなら、そうしなよ。 - If you think that's fine, then go ahead and do it.