"I am lying beside the window."
Translation:Az ablak mellett fekszem.
Ok, so this one rather annoyed me. It seems to me that many of the sentences in this section could be translated using the -nál/nél construction, or with another construction such as mellett. Indeed, I think that "by" translates better with mellett, although in reverse it's true that "-nál" sometimes translates best as "by".
Why doesn't this one accept "mellett" or "-nál"?
No, for the same reason that "I am lying the window beside" does not work in English.
"beside" is a PREposition, which means that it stands BEFORE its noun: "beside the window" (first "beside", then "the window").
mellett is a POSTposition, which means that it stands AFTER its noun: az ablak mellett (first az ablak, then mellett).
English has one common postposition: "ago". We say "I moved here three years ago." But we can't say "I moved here ago three years" -- that's like saying mellett az ablak.