Great video by Alex Miller about Esperanto’s correlatives. For me, this was one of the most challenging parts of learning Esperanto. I understand them now, but it took a while and a lot of thinking and studying. I found a good book that helped a lot, too.
In all honesty, the way that I finally mastered correlatives was taking a mini-course on Memrise about them. Also, I began writing Esperanto daily and found that I had to know how to use a correlative, such as “kiu,” in order to write properly. So, I learned by study and by practice.