What is the motivation behind this sudden change?
这种突然转变背后的动机是什么？
The sudden noise made her start.
突如其来的声音吓了她一跳。
His heart gave a sudden leap when he saw her.
他看见她时，心猛地一跳。
The children were surprised by the sudden entry of their teacher.
老师突然进来使孩子们感到意外。