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[书面] [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. 我为音乐而生，对音乐的不懈探索已经成为我生活的动力。

You begin to understand what actions were the mainspring of the story. 你开始明白什么是这个故事的主要情节了。