"I thank the daughter."
Translation:Ich danke der Tochter.
You'll hear this with the pronouns as well: "Ich danke dir." "Ich danke ihm." Kind of like you're giving thanks to [you/him], so it takes dative.
Make sure you always read the tips!
(Look for the light bulb icon when starting a lesson - may not be available on all platforms, but definitely is on the web!)