why didn't the answer 'other thing' work? isn't it just like 'another thing'?
Jiná is another and different how to know which?
jiná - different
další - next
"Another", however, can mean both "next" and "different".
but why czech has stres sign on every word
This is not a stress sign. Czech diacritics mark completely different letters with different pronunciation. Please do read the Tips and notes of the first Skills. They contain an introduction to Czech orthography and phonology.