What is the difference between “senão” and “se não” in Portuguese?

Senão written as one word means “otherwise” or “or else,” and it introduces a consequence: Fale baixo, senão eu vou reclamar, “Talk quietly, or I’ll complain.” Written as two words, se não, it means “if not”: se não chover, vamos, “if it doesn’t rain, we’ll go.” Native speakers mix them up in writing, so seeing the difference already puts you ahead.

The cleaning lady warns the noisy class twice with senão: “Senão eu vou contar pra professora” and “Senão eu vou mandar vocês limparem todas as salas.” Each time the threat follows right after the order to stop. A quick test: if you can replace it with “otherwise,” write it as one word. Also note the friendly-strict pra for para in speech.