突然 / 忽然 / 猛然【tūrán / hūrán / měngrán】

突然, 忽然 and 猛然 all mean "suddenly". As adverbs they go before the verb, and 突然 and 忽然 can also open the clause, before the subject.

突然 is also an adjective

忽然 is only an adverb. 突然 can also be the predicate after 很 or 太, or describe a noun with 的. When "sudden" is a description and not a manner, only 突然 works.

• × 这个消息很忽然 → 这个消息很突然
• × 忽然的变化 → 突然的变化

As adverbs, the two are close. 忽然 is a little softer and more literary, at home in stories; 突然 stresses that it came without warning.

猛然 – sudden and forceful

猛然 adds force. It goes with quick physical movements – 站起来, 回头, 抬头 – and with 想起 when a thought hits hard. It belongs to written narrative, and it can't be an adjective.

Weather and slow changes don't take 猛然.

• × 天猛然黑了 → 天突然黑了