应该添 yetyet是‘然而在此基础上...’while是比较两的东西，比如：I was watching tv, while Melissa was doing her homework.however用于转折，不是比较。用however的时候，前面诉说一件事的一方面+however+这件事的反面.解释的不是太好。主要得靠感觉
