I don't know how they explain how Korean works at all. But I could correct what you've done wrong in the sentence. If you want to say, I studied Korean today, you could say 난 오늘 한국어를 공부했어요. If you want to say, I study Korean everyday, you could say 난 매일 한국어를 공부해요. As you can see, both come from the basic form of 공부하다. That means, If you want to use 공부 as a verb, You have to combine 공부 and 하, 해 or so first and then, 었, 웠 or so can follow it. I hope this comes across well.