楼下(lóu xià) - downstairs, the floor below in Mandarin

楼下 (lóu xià) means downstairs, and by extension the ground floor of your building, the street door, or simply the flat below yours. It is the everyday partner of 楼上.

Mandarin treats it as a place noun, not an adverb. It takes a preposition and sits in front of the verb: 我在楼下等你, 他到楼下去了, 楼下有人. English lets you tack "downstairs" onto the end of a sentence, and copying that word order gives you 我等你楼下, which nobody says.

The other useful habit is that 楼下 can be the subject in its own right. 楼下很吵 means the people below are making a racket, and 楼下的 is a neat way to refer to whoever lives there.