I looked up the difference of 그 and 저. It's pretty simple and times like these I wish Duolingo would put in a quick note to tell us the difference. 그 is something already spoken about whether it's there or not, 저 is something you can see now but is too far away to touch.
Korean has three different demonstrative categories while English really only has two, which can make it harder to understand the difference. 이 is close to the speaker (this), 그 is close to the listener (that) and 저 is far from both of them (that over there).
Can anybody tells me why I should use vocab 크다 instead of 높다? I'm a bit confused because, in the lesson's description, there is an information that tells us if we are talking about the size of the building, trees etc we should use the word 높다.