Questions & Answers about Ktoś kiedyś ukradł ten obraz.
How do the words ktoś and kiedyś get their meaning from kto and kiedy?
The suffix -ś turns question words into indefinite words, meaning the exact person or time is unknown. Kto means who, so ktoś becomes someone. Kiedy means when, so kiedyś becomes sometime or once.
Since we don't know who stole the painting, why is the verb ukradł in the masculine form?
In Polish, the pronoun ktoś (someone) is always treated as grammatically masculine. Even if the unknown thief happened to be a woman, you must use the masculine past tense form ukradł to match ktoś.
What case is ten obraz, and why didn't the endings change?
It is in the accusative case because it is the direct object (the thing being stolen). However, obraz is a masculine inanimate noun. For these types of nouns, the accusative form looks exactly the same as the basic dictionary form (the nominative). This rule applies to the demonstrative word ten as well.
Could I change the order and say Kiedyś ktoś ukradł ten obraz?
Yes! Polish word order is very flexible. Starting with Kiedyś would put a little more focus on the timing (Once, someone...), while the original order Ktoś kiedyś feels very neutral and standard.