난 모든 것을 들었다. 

I heard everything. ✔​

나중에 보자. 

See you later. ---> I will see you later.

=> This sentence is okay. This is another way of saying it. ​

Marty는 공원에 있는 오리에게 먹이를 주고 있다. 

Marty is feeding to the park duck. ---> Marty is feeding the ducks at the park.

=> Corrected : The sentences were corrected grammatically. ​​