mainspring

1. N-COUNT If you say that an idea, emotion, or other factor is the mainspring of something, you mean that it is the most important reason or motive for that thing. 主要部分[書麪] [usu sing, usu 'the' N 'of' n]

My life has been music, and a constant search for it has been the mainspring of my life. 我爲音樂而生，對音樂的不懈探索已經成爲我生活的動力。

You begin to understand what actions were the mainspring of the story. 你開始明白什麽是這個故事的主要情節了。