This post was inspired by a question that someone asked on the /r/korean subreddit (I lurk around there sometimes; if you Reddit, please do check out that sub!). The question was why some verbs use -고 있다 and some use -아/어 있다. If you check the question link, you can see my short answer on the original question. Also, I’ve actually gone over these two in the past, in a post about the progressive tense. The purpose of this post is to sort of clarify the difference between the two with a bit more explanation and contrasting examples.