I wrote Frühstuck instead of Frühstück and Duolingo told me to be careful not to do this confusion. What does Frühstuck mean?
Nothing. It's just spelt wrong.
why is it dem here... i am not getting whats dative here & how
"nach" + Dativ
which prepositions goes with dativ and which with akkusativ?
Dative here: https://en.wikipedia.org/wiki/Dative_case#German
Accusative here: http://german.about.com/library/blcase_acc2.htm