"The hotel is expensive."
'이' is the subject mark particle, the word cannot be right if you do not choose it with '이'.
how do i know when it should be 이/가 vs 은/는? from what i know there is just a difference in what you're emphasizing or something like that so there isn't one that is wrong and one that is right
i think 은/는 is used when youre speaking about something more generally and 이/가 is more specific. it's better explained in the notes duolingo has in one of the first few lessons