Why do Polish words sometimes use an ogonek?
For example, sometimes “a girl” is “dziewczynka” or “dziewczynką”. Why is this?
Feminine nouns ending in -a in the nominative end in -ą in the instrumental - which is why you'd say Dziewczynka je, but Ona jest dziewczynką (as być takes the instrumental).
There is a pretty good discussion about polish declination here: https://www.duolingo.com/comment/14379227/Polish-Cases-chart (you can find links to useful tables in that thread)