Breakdown of Po co sam kupiłeś ten stary bunkier?
ten
this (m)
stary
old
sam
alone / himself (emphatic) / pure / nothing but
kupić
to buy (perfective)
Questions & Answers about Po co sam kupiłeś ten stary bunkier?
Why is Po co used here instead of Dlaczego?
Po co asks about the purpose or goal of an action (what for? what will you do with it?). Dlaczego asks about the cause or reason in the past (why did this happen?). Here, the speaker is asking about the buyer's future plans for the bunker, so po co is the natural choice.
Why does the sentence use sam instead of samemu?
Sam is an adjective that agrees with the subject. The verb kupiłeś shows that the subject is a single male (you), so it takes the masculine nominative form sam (by yourself / alone). Samemu is the dative form, which is only used in specific grammatical situations (like trudno mi samemu - it is hard for me alone), not for the main subject of an active verb.
Why does ten stary bunkier look like it is in the nominative (dictionary) form if it is the object being bought?
It is actually in the accusative case because it is the direct object of the verb kupiłeś. However, bunkier is a masculine inanimate noun. For masculine inanimate nouns, the accusative case always looks exactly the same as the nominative case, so the words ten stary bunkier do not change their endings.
Could I move sam to the end and say Po co kupiłeś ten stary bunkier sam?