さもないと

さもないと means "otherwise", "or else". It starts the second sentence: the first says what someone should do, and the second says what goes wrong if they don't.

The first sentence is an instruction, advice or an obligation – something that can still be done. The second is an unwelcome result, not a neutral alternative.

Register

さもないと is formal and a little stern. You meet it in writing, in warnings and in a parent's or teacher's lecture. In ordinary conversation, そうしないと or でないと says the same, and じゃないと is the casual form.

さもなければ means the same and is more formal still, at home in written instructions and contracts. さもなくば is older and literary.