1. 楼梯
    Stairs are a set of steps inside a building which go from one floor to another.

    e.g. Nancy began to climb the stairs...
           南希开始爬楼梯。
    e.g. We walked up a flight of stairs...
           我们上了一段楼梯。

2. 一层楼梯;一段楼梯
    A stair is a flight of stairs.

    e.g. I followed her down the stair.
           我跟着她下了楼梯。

3. 一级楼梯
    A stair is one of the steps in a flight of stairs.

    e.g. Terry was sitting on the bottom stair.
           特里正坐在最下面的一级楼梯上。