"A mountain is not at the sea."
Translation:산은 바다에 없습니다.
Not an expert but from what I know 없습니다 is present tense and 업섰어 is past tense. I think it has to do with formal and informal speech as well but I'm not sure
는 and 은 are both topic particles. 는 is used when the word ends in a vowel sound while 은 is used when the word ends in a consonent. Similarly 가 and 이 are subject particles when the word ends with a vowel and consonent, respectively.
Before it said 바다 means ocean and now it's sea so does it mean both? Is there a difference?