upstairs用作名词时的意思是“楼上”,是单数名词,多与the连用。

The ground floor needs painting, but the upstairs is nice.一楼需要油漆了,楼上还相当好。
The upstairs of the house is totally furnished.房子楼上部分家具已备齐了。
Suddenly they heard a cry from upstairs.突然他们听到楼上一声喊叫。

upstairs用作形容词意思是“楼上的”,在句中只用作定语。

用作定语
▲～+n.
He is waiting in the upstairs hall.他正在楼上的大厅里等候。
He lodges in an upstair room.他住在楼上的房间。
The downstairs rooms are not as quiet as the upstairs ones.楼下房间没有楼上房间安静。
The neighbors in the upstairs flat overlook our garden.住在楼上套房中的邻居可以俯瞰我们的花园。
He has gone to one of the upstairs rooms.他已经到楼上的一个房间里去了。
Neighbours watched from their upstairs windows.邻居们从他们楼上的窗户往外看。

1.upstairs用作副词时意思是“在楼上,往楼上”。
2.upstairs常修饰动态动词,在句中可用作状语(常置于句末)或表语。
3.upstairs无比较级和最高级形式。

用作状语
We went upstairs to bed.我们上楼去睡觉。
It was very hot. I went upstairs and changed into a thinner dress.天气非常闷热,我上楼换一件单薄的衣服。
Abel asked to be excused from accompanying them and went upstairs to her room.艾贝尔请求不要让她陪伴他们,然后就上楼回到自己房里去了。
We all hurried upstairs to see what was going on.我们都慌忙上楼看看发生了什么事。
I don't want any of you children crashing about upstairs while your father's asleep.你爸爸睡着了,你们这些孩子谁也不许在楼上蹦蹦跳跳。
He's upstairs taping; don't make too much noise or else the machine will recorded it.他在楼上自己录音呢,噪声别太大了,要不然那机子也会把它给录进去的。
用作表语
Come up! I'm upstairs.上来! 我在楼上。
The tea things are upstairs, could you carry them down for me?茶具在楼上,替我拿下来好吗?
My pen and books are upstairs; please bring them down.我的书和笔都在楼上,请给我拿下来。
The bedrooms are upstairs and the dining room is downstairs.寝室在楼上,餐厅在楼下。