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(wèi)音樂而生，對(duì)音樂的不懈探索已經(jīng)成爲(wèi)我生活的動(dòng)力。

You begin to understand what actions were the mainspring of the story. 你開始明白什麼是這個(gè)故事的主要情節(jié)了。