〜なければならない or 〜なくてもいい?

Obligation, and its exact opposite

Each one below is our full write-up of that form, in the order worth reading them. Nothing is summarised — you are looking at the same pages the grammar index links to, put next to each other.

〜なければならない

must, have to

V-ない stem + なければならない / なくてはいけない

Obligation. ならない leans toward a general rule; いけない leans toward this particular situation, though in practice they are near-interchangeable.

Spoken Japanese contracts heavily: なきゃ, なくちゃ. Recognising those in the listening section matters more than producing them.

明日までに出さなければなりません。I have to hand it in by tomorrow.
薬を飲まなくてはいけません。You have to take your medicine.

〜なければならない on its own

〜なくてもいい

don't have to

V-ない stem + なくてもいい

The absence of obligation — the direct counterpart of なければならない.

Don't confuse it with 〜てはいけない (must not). "You don't have to come" and "you must not come" are opposite messages built from similar parts.

明日は来なくてもいいです。You don't have to come tomorrow.
急がなくてもいいですよ。There's no need to hurry.

〜なくてもいい on its own

〜なければならない or 〜なくてもいい?