Breakdown of W jakiej partii jest ten poseł?
być
to be
ten
this (m)
w
in / on (a day)
jaki
what kind of, what ... like (asks for an adjective)
Questions & Answers about W jakiej partii jest ten poseł?
Why use jakiej here instead of której if the English translation says "Which party"?
In Polish, który (which) implies a specific choice from a small, restricted set of options (like pointing at a ballot and asking "Which of these?"). Jaki literally means "what kind of," but Polish speakers naturally use it when asking for someone's general affiliation, like asking "What party is he a part of?" out of all the possible options out there.
Why does partii end in a double -ii in the Locative case here?
The base dictionary form is partia. As a general spelling rule, feminine nouns ending in -ia that are borrowed from foreign languages (like partia, historia, or armia) take a double -ii in the Genitive, Dative, and Locative cases. Native Polish words ending in -ia usually take a single -i.
In English we can say "Which party is this MP in?". Can we put the preposition w at the end of the sentence in Polish?
No, Polish never allows you to leave a preposition hanging at the end of a sentence. A preposition like w must always stay glued directly to the noun phrase it modifies. Therefore, you always have to phrase it as "In which party..." (W jakiej partii...).
What happens to the word poseł if the MP is a woman?