Translation:This boy and this girl are a great couple.
The difference lies in the words "to" or "jest". If you use "to", it is Nominative (nouns and adjectives both ending -a in fem. sing). If you use "jest" it is Instrumental (ending nouns -ą, and adjectives -ą).
Example: X to Y = Zuza to piękna kobieta = Nominativ
X jest Y = Zuza jest piękną kobietą = Instrumental
But watch out, "To jest (add noun here)", (This is ...) will still be Nominative.
Not a native but fellow learner, hope this helps and if anything is wrong or misleading, pls correct.
I don't know the right place to ask this question, but would it be possible to develop a feature for Dualingo where you go from hearing X language straight to writing in Y language? Example: You hear "Jestem mężczyznę" and have to write "I am a man." I find myself writing the sentence, then rereading it to translate, whereas this way I feel would force you to translate as you listen. Just an idea I had.