Why not "indoor"? Does it have to be used in plural form?

Native speaker’s answer
Rebecca
Ah, so there's a slight difference between "indoor" and "indoors," which doesn't relate to singular and plural forms. "Indoor" is an adjective that means something done or used inside a building. "Indoors" is an adverb that means into or within a building. It's also a noun that means the area or space inside a building. Ex: We're playing indoor basketball. = We're playing the type of basketball played inside a building. Ex: We're playing basketball indoors. = We're playing basketball inside a building. Ex: This is an indoor plant. => plant meant to grow inside Ex: I struggle to grow plants indoors. => difficult to grow plants inside.