突然 vs 忽然

[woliveri]

Can someone tell me the differences between these two?

When to use each?

Thanks

[semantic nuance]

They are interchangeable.

Hope it helps!

[randall_flagg]

my teacher told me 突然更忽然一些。。。 (or was it the other way around? darn it!!!) anyway, he also says that they are interchangeable, yet one of the two is even more sudden.

[ruiskukka]

"突然" is an adjective. "忽然" is an adverb.

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1. 突然 + noun

(Right) 情况发生了突然的变化。

(Wrong) 情况发生了忽然的变化。

2. "很"/"太"/"十分"/"非常" + 突然

突然 + "极了"/"得很"

(Right) 天气的变化很（太、十分、非常）突然。

(Wrong) 天气的变化很（太、十分、非常）忽然。

(Right) 天气的变化突然极了（得很）。

(wrong) 天气的变化忽然极了（得很）。

3. 动词+得+（......）突然

(Right) 他的病来得有点儿突然。

(Wrong) 他的病来得有点儿忽然。

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Interchangeable:

(Right) 我忽然间想起他了。

(Right) 我突然间想起他了。