"很突然" 可以用英语表示为 "suddenly" 或 "out of the blue"。

"suddenly" 是一个常用词，意思是 "突然地"，"without warning"。
例如：

Suddenly, the pghts went out. (突然间, 灯灭了)
She suddenly reapzed that she had left her keys at home. (她突然意识到她把钥匙留在家里了)

"out of the blue" 是个习语，意思是 "突然的、出乎意料的"。
例如：

He called me out of the blue yesterday. (他昨天突然打电话给我)
I got a job offer out of the blue last week. (我上个星期突然得到了一个工作机会)

总之, "suddenly" 和 "out of the blue" 都可以表示 "很突然" 的意思。