This pattern tells someone that they don't have to do something. It's the opposite of なくてはならない (must) and is often used to give permission or reassurance, like saying 'You don't need to...' or 'It's okay if you don't...' in English. For example, if someone asks if they must come to the meeting, you can say 来なくてもいいです (You don't have to come).

Form it by taking a verb in its negative ない-form (like 行かない), dropping the い, adding く (so 行かなく), and then attaching てもいい. The も is often kept in standard Japanese. With な-adjectives and nouns, use じゃなくてもいい (e.g., きれいじゃなくてもいい 'It's okay if it's not pretty', 学生じゃなくてもいい 'You don't have to be a student').

This is a common, polite way to say something is optional. It's not rude, but it's not overly formal either—it's used in everyday conversation. You can use it with friends or in polite speech by adding です.