Sylvan J Zaft (1935 – 2015) was a very active Esperantist who lived and taught in Detroit.   After his death Dr. Zaft’s Esperanto publications were assembled on the celebratory website,   

Many years earlier Dr. Zaft had generously given permission for the publication of his very readable discussion of Esperanto’s qualities and difficulties for English speakers – Esperanto – A Language for the Global Village.  Later he made this book and some other items available on Amazon’s Kindle, where they still can be very easily and  cheaply consulted. 

Sylvan Zaft on Amazon Kindle

The correlatives seem to be a new kind of thing for many students of Esperanto.  For one thing the students have never encountered a special Chart of the Correlatives in any other language.  Most of them have never learned the meaning of the word correlative in their own language.