"There is a large forest near the water."
Translation:물 가까이에 큰 숲이 있어요.
It's probbably considered incorrect because of the lack of the location particle ~에. Though I think it would be a perfectly fine sentence in everyday speech.
Yes. The particle 이/가 marks the subject and you can put the subject at the back or front of the sentence. However duolingo doesn't seem to agree. I had native teachers though and they all said it was fine