"여자가 물을 마십니다."
Translation:The woman drinks the water.
That is one tough question.
Until someone else shines in with a comprehensive answer in this post (or as a separate post to be stickied in the forum), I'll just give you this link to the countless discussions you can find about this on Korean language learners blogs.
Also, I do not understand all the finesses myself either on this topic.
For now, I just try to remember whenever someone corrects me and tells me the form I used sounds unnatural.