①used to say what will happen if another thing does not happen

否则，要不然

We must be there by six, or else we'll miss the beginning.

我们一定要在6点前赶到那里，要不然就会错过开场。

②used to compare two different things or situations

(用于对比两种不同的事物或情形)要不，要么

She's either really talkative and you can't shut her up or else she's silent.

她要么滔滔不绝，你都堵不上她的嘴，要么就一言不发。

③used as a threat, sometimes humorously

(表示威胁，有时是一种幽默的说法)要不然，否则的话

He'd better find it quickly, or else (= or I will punish him in some way)!

他最好马上找到它，要不然，哼!