#3 Even better—it depends on the building. Some buildings in the US do have a ground floor and a first floor. I think it’s usually buildings with multiple entrances, where you can enter onto the ground floor from one side of the building or the first floor from the other (usually because it’s on a hill or something), but I’m not sure if that’s always the case. But if a building doesn’t have both, the terms “ground floor” and “first floor” are used interchangeably. Sorry about that. On the plus side, most buildings rooms’ are numbered so that the first floor is the 100s, the second is the 200s, and so on, and if the ground floor is separate from the first, it’ll usually be in single digits and may have a G in front (like G-07). We don’t even bother to say what floor things are on most of the time, because of that.