This is the 461st most frequent Chinese word.
突然
Suddenly; abruptly.
"突然" is used here as an adverb modifying the verb "下起" to emphasize the unexpected commencement of rain.
天突然下起了雨。
It suddenly started raining.
"突然" is used here as an adjective to describe the unexpected nature of "离开," explaining the reason for others' surprise.
他的突然离开让大家都感到惊讶。
His sudden departure surprised everyone.
"突然" functions here as an adverb to introduce an unexpected event while the speaker was engaged in an ongoing activity.