play by ear
1．play a musical instrument by remembering the tune凭听觉记忆演奏
She plays the piano entirely by ear．她不用乐谱，完全凭听觉记忆演奏钢琴。
2．decide what to do as one goes along酌情而定
We don't know what may crop up，so you'll have to play it by ear．我们不知道会出现什么情况，因此你得见机行事。