By default, Inform doesn’t recognize the names of kinds. X DOOR won’t work unless you (or the library) explicitly Understand “door” as a door.
However, it does recognize the plural names. X DOORS does work straight out of the box…and always fails, because examining can’t take multiple objects. The intent is presumably for indistinguishable objects: if you make five coins, TAKE COINS should take all of them instead of just one. But it has the side effect of making the word inherently and irrevocably plural: if you have an object in your game called “a pair of double doors”, there’s no possible “does the player mean” rule that will let you unambiguously X DOORS. (Worse yet, this means the words “container” and “supporter” are always recognized and always plural, since the dictionary only stores the first nine letters of a word: “containers” and “supporters” are ten letters long.)
My standard solution is to say “The plural of door is asdfasdf” and so on for all the other kinds I want to override. But this is very clearly a hack, and also means a pair of unnamed doors will be described as “two asdfasdf” in room descriptions.
How difficult would it be to simply remove this behavior from the compiler? I haven’t been able to find where exactly this is implemented. If authors want a particular word to be understood as a plural, that’s still possible (Understand “coins” as the plural of a coin)—I just find the compiler’s automatic handling to be undesirable more often than not, and there’s currently no clean way to disable or override it.