なさい is a command form that is softer than the abrupt imperative (like 食べろ) but still firm. It is commonly used by people in authority, such as parents talking to children or teachers to students. It can sound a bit harsh if used among peers, so it's not typical in casual conversation unless joking or being emphatic.

You form it by taking the ます-stem of a verb and adding なさい. For example, 起きます (to wake up) becomes 起きなさい. The pattern is very straightforward, but remember that the stem is the part before ます, so for い-verbs like 話します, the stem is 話し, giving 話しなさい. For る-verbs like 食べます, the stem is 食べ, giving 食べなさい.

なさい is also used in set expressions like ごめんなさい (I'm sorry, literally 'do forgive') and おやすみなさい (good night, literally 'do rest'). In these cases, the なさい has become part of a fixed polite phrase.