Translation:The small boy eats a big banana.
singular or plural doesn't have anything to do with it. In this case if you put klein in front of the word it describes it will always be kleine but if you use it in a different way it becomes klein. for example Het kleine huis ( the small house) Het huis is klein ( the house is small) de kleine huizen ( the small houses) de huizen zijn klein ( the houses are small)
theres a site called memrise ...basically like flashcards with pictures and stuff. might help a little. and try to practice them all in context. and maybe try to connect it to english if you have to - kleine, k, kid, kids are small ...also the dutch word for clean is schoon