I think you mean 'steps.' We think of 'stairs' as being inside a building, not in the back yard. Anyway, 'stairs' is plural and accordingly you must use 'are.' E.g. The steps are in the back yard. The stairs are through the hall.

simple answer: "stairs" = plural therefore use "are" For reference: "steps" is normally used for outside, but also: "watch your step" = be careful where you are walking "our child has taken his the first step" - starting to walk instead of crawl. "the first step is high, but the rest of the steps are low" - singular & plural "Learning languages is hard, but take one step at a time"

"Stairs" is treated as plural. "The stairs are in the backyard". You can use the word "stair" to mean a single step, or to refer to the entire set of stairs. You can also say "staircase", "stairway" or "flight of stairs".

"The stairs are in the backyard" would be correct, since "stairs" are plural