"저는 물을 마십니다."
Translation:I drink water.
In Korean, there's a separate form for the progressive tense. Duolingo is treating it as if the present tense is never used for the meaning of present progressive, but in reality, Korean speakers use present tense that way all the time.
They just use the progressive form when needed to clarify things.
을/를 is the object marker, marking a thing as having an action done to it. When you drink water, the water is a passive thing with something being done to it, so it's definitely an object and so 물을 absolutely makes sense.
I'm a sentence that uses "물이", no action would be occurring on the water.
Like, in "집에 물이 있습니다." (There's water at my house.), nothing is happening to the water. It just exists.
Wow, you're the first person I encounter here that says this course is easy x.x
Btw, I understand what Sena's doing, if there's a bahasa Indonesia course for English speakers I'd like to try to even if I'm a native. I might contribute in the comments and in giving alternative answers to the course developers. I've seen some native Koreans helping in the comments too here.