You probably know this by now, but for new users who might be wondering...
You don't really choose to use a hard or soft adjective.. When you learn a new adjective, you also learn whether it is hard or soft. When you use a particular adjective in a sentence, you use the correct endings for that adjective, depending on whether it follows the hard pattern or the soft pattern. More information is available at this handy site: https://en.wikipedia.org/wiki/Czech_declension#Adjective.