他对这个理论只有一个模糊的概念。

He only has a vague ___notion about this theory.

他对幸福的看法很单纯。

His ___notion of happiness is quite simple.

她对成功的概念与大多数人不同。

Her ___notion of success is different from most people's.

她突然有个念头想学弹钢琴。

She had a sudden ___notion to learn how to play the piano.