This factor is the Chinese writing system (which Japan borrowed and adapted centuries ago). The learner needs to know at least 3,000-4,000 characters to make sense of written Chinese, and thousands more to have a real feel for it. Chinese, with all its tones, is hard enough to speak. But the mammoth feat of memory required to be literate in Mandarin is harder still. It deters most foreigners from ever mastering the system—and increasingly trips up Chinese natives.