This “verb stem + なさい” is used for parents talking to children, teachers to students, test directions or any other situation where a command is to be followed by a subordinate.

Because of this construction, these “commands” can only be given in the affirmative.

うちに毎日電話しなさい。　- うちにまいにちでんわしなさい。　- Call home every day.

かっこの中に単語を入れなさい。　- かっこのなかにたんごをいれなさい。　- Fill in the blanks with a word.

先生の話をよく聞きなさい。　- せんせいのはなしをよくききなさい　- Listen to what the professor has to say.