A user has contributed the following comment several times, except in each case they included extraneous and non-substantive content not consistent with Duolingo's community guidelines. I am quoting from this user here (without stating their nick, to avoid a "call-out"), as their substantive comment (for which thanks are due!) may help some users struggling with the pronunciation of ď and ť:
Ť and Ď are the palatal stops /c/ and /ɟ/. For Czech people, they are soft versions of T and D, but acoustically speaking and depending on your native language, they could also be interpreted as soft versions of K and G. Palatalization means that a sound moves closer to the hard palate of your mouth. T and D are alveolar stops produced in the front of your mouth. Palatalization pulls them back. K and G are velar stops produced in the back of your mouth. Palatalization pulls them forward. The result is acoustically similar in both cases.
Please read about Czech orthography and pronunciation in the Tips and notes for the very first skills. Those are not K and G, but Ť and Ď. If the Tips and notes are not sufficient, proceed to https://en.wikipedia.org/wiki/Czech_phonology and ask for some clarification if it is not clear.