Ok, but I was thinking that if it's the object of the sentence i.e. 'the room' which is being cleaned, and that then takes the Accusative, then surely the word 'your' must also take the Accusative, so I wrote 'twojego'.
How about: "Kiedy sprzątałam twój pokój znalazłam nóż"? - on the basis that the knife is the key word of the sentence? (It was rejected). And can a sentence ever start with "gdy" (I can't recall ever seeing gdy in that position)? Thanks.
Yes, such a sentence can start both with "kiedy" and with "gdy". The difference between those is that "kiedy" is used for asking questions (When are we going to the movies?) but "gdy" cannot be, it wouldn't make sense.