1. 往楼下;顺楼梯而下;至楼下
    If you go downstairs in a building, you go down a staircase towards the ground floor.

    e.g. Denise went downstairs and made some tea.
           丹尼丝下楼泡了茶。

2. 在楼下
    If something or someone is downstairs in a building, they are on the ground floor or on a lower floor than you.

    e.g. The telephone was downstairs in the entrance hall...
           电话在楼下的门厅里。
    e.g. Everybody was downstairs watching a movie.
           大家都在楼下看电影。

3. 在楼下的;在一楼的
    Downstairs means situated on the ground floor of a building or on a lower floor than you are.

downstairs是什么意思

    e.g. She repainted the downstairs rooms and closed off the second floor.
           她重新粉刷了楼下的房间，并把二楼封上了。

4. 楼下;底层
    The downstairs of a building is its lower floor or floors.

    e.g. The downstairs of the two little houses had been entirely refashioned.
           两栋小楼的底层已经彻底翻修过。