Knows that "while" was once a noun in the good ol' days when English was still declined.
True - and, as far as I can tell, the use of while as in "While I was mowing the lawn, the cat was chewing the power cord" didn't exist. (You'd have to use the Old English form of "when", which was "ða" (pronounced "thah"), as in "When I was..." etc.) Bear in mind, though, that "while" does still work as a noun, as in "It was a while before I could find the charred remains of the kitty."