In this case it would be right to say "Dies ist ein Test" if you wanted to translate:"this is a test." In short: there are situations, where you can translate 'das" into "this". But the great bulk of contexts should be translated into "that". Start learning the major rule and pick up the exeptions on that rule as you get more proficient. And don't take duolingo's word for it. By keeping to the major rule you make it right and also easy. In my experience keeping things simple in the early stages really pays of. You remember things quicker and later on you will have a framework for the exeptions etc.