Can you explain, please, why I appear to be using genitive (or accusative for masc animate) after the word 'coś'? Thanks. 06/03/18 - I've eventually found the answer in Polish: A Comprehensive Grammar By Iwona Sadowska.( Para 2.4.5). I'm no good at forwarding links, but put into search engine 'how to decline the polish word ’coś’ and a full explanation came up. Thanks for all your work on Duolingo.
Yes it is. Note that while in "[On/] pokazał mi..." you cold omit the pronun, you can't do it with "On mi pokazał" because you can't start with "Mi".
I put in "on pokazał mi coś bardzo ważnego" and my answer was still incorrect.