"The fast running student eats a lot of rice."
Translation:빨리 달리는 학생이 밥을 많이 먹습니다.
Yup! 밥을 많이 먹다 is the most natural.<h1>Grammar</h1>
많이 is an adverb, which means 밥을 많이 먹다 is parsed as:
- [rice] [eat lots] (eat rice a lot)
많은 is an adjective, which means 많은 밥을 먹다 is parsed as:
- [lots rice] [eat] (eat lots of rice)
The former describes of the degree of the action and the latter describes of the degree of the noun. Koreans generally think of eating a lot rather than lots of something being eaten.