He used to be a slow learner. When he turned 15 he suddenly was enlightened and became a quick learner.
enlightened (by God)白話的翻出即可.
他本來學習很慢, 但是一過15歲以後突然開竅, 學習快了起來.
She is shy, but when there is a guest at home, she becomes exited and hyperactive and out of control.