끝내다 = finish
됐다 - This has so many different meanings.
be, become, turn into, come, reach, develop into and etc. You have to see the entire sentence to know the exact meaning of it.
a : i just bought a brand new notebook. 나 노트북 새로 샀어. (informal way.)
b : O! what for ? 뭣 때문에?
a : for study,maybe. 공부 하려고. 아마도..