Questions & Answers about Po czternastu dniach mam odpowiedź.
Because the preposition po here means after, and in this meaning it requires the locative case.
So both words change form:
- czternaście → czternastu
- dni / dictionary form dzień → dniach
That is why you get:
- po czternastu dniach = after fourteen days
Using po czternaście dni would be ungrammatical in this sentence.
Because Polish numerals can decline too, just like nouns and adjectives.
Here the numeral is part of a phrase governed by po, which requires the locative. So czternaście takes its locative form:
- nominative: czternaście
- locative: czternastu
This is very common after prepositions that require a specific case.