Translation:I found a knife when I was cleaning your room.
The further down the language trees I go, the more sinister Duo becomes....
Could be that your teenage daughter eats in her room and forgets to take her cutlery back to the kitchen. If was found THE knife then that would be more worrying.
In place of "when", "while" is acceptable but "whilst" is not. Is this true?
Response: I found a knife when I cleaned your room. Why is this answer also not correct?
Because "sprzątałam," the past imperfect form, is used. This means "I was cleaning" and that the act occurred in the past but does not indicate that the action was completed. Whereas "posprzątałam," the past perfect form, translates to "I cleaned" and indicates that the act occurred in the past and was completed. Thus, "Znalazłam nóż, gdy posprzątałam twój pokój" would better correspond to your response.
Well, as I was told by some English natives, even simple "cleaned" could be interpreted as an imperfective verb (although it looks totally perfective to a Polish eye), and thus it's also a correct translation. Although "was cleaning" is surely better. Unfortunately this is not that easy and straightforward as we (I) would like it to be...