Thus, you have successfully installed the curtains inside your living room, not outside. Give yourself a nice pat on your back (it is underneath your shirt).
The article. Le and les are pronounced differently even when the noun is not.
It's arbitrary--like "pants" or "scissors," which are often treated as singulars in other languages (these are known as "plurale tantum" when they're treated the way that is done in English).
Curtain(s) doesn't fall into this category. It is a countable noun with a singular and a plural.
Most single windows have two curtains - one on each side. That's why we almost always say curtains. Sometimes a window is so narrow that it can only accommodate one curtain. With such a window, we could use this exact sentence. "The window is behind the curtain."
I have two such windows in my living room. The bay window in my dining room however is so wide that it is behind curtains, not a curtain.