表示选择，意为“或”“还是”：

Is the radio off or on? 无线电关上了还是开着的?

Would you prefer tea or coffee? 你喜欢茶还是咖啡？

Is he asleep or awake? 他睡着了还是醒着?

Are you going to America by boat or by air? 你到美国是坐船还是坐飞机？

You may go or stay, according as you decide. 是去是留由你自己决定。

Are you from North China or South China? 你是华北人还是华南人?

You can come now or you can meet us there later. 你可以现在来，也可以稍晚和我们在那里碰头。

表示一种否定的条件，意为“否则”：

Come on, or we’ll be late.  快点，否则我们要迟到了。

Hurry up, or you'll be late for school. 赶快，否则你上学就要迟到了。

Dress warmly, or else you’ll catch cold. 穿暖和点，否则你会感冒的。

Be careful, or you’ll break that vase! 小心，否则你会把那花瓶打碎!

Cross the road very carefully. Look both ways, or you might be knocked down. 过马路要非常小心，要看两边，不然会被车撞倒。

可表示“要不就是”：

He must be joking, or else he’s mad. 他一定在说笑话，要不就是疯了。

The book must be here, or else you’ve lost it. 这书一定在这儿，要不就是你丢失了。

用于否定句中代替and。

He was not clever or good-looking. 他不聪明，也长得不好看。

比较：

They sang and danced. 他们既唱歌又跳舞。

They didn't sing or dance. 他们既没有唱歌也没有跳舞。

用于习语

The work is more or less finished. 工作大体上完成了。

They consist of 1700 or more tribes. 他们由1700个或更多部族构成。

There’s just one or two details I want to make sure about. 还有一两个细节我想弄清楚。

Either your mother or your father may come with you. 你母亲或是你父亲会陪你去。