突然 vs 忽然

Both drop in as "suddenly" adverbs (我忽然想起); only 突然 also works as an adjective — 很突然 is fine, 很忽然 is not.

突然

tūrán ㄊㄨㄖㄢˊ

sudden; abrupt; unexpected

我突然想不起她的名字。

I suddenly can't remember her name.

忽然

hūrán ㄏㄨㄖㄢˊ

suddenly; all of a sudden

我忽然想起了我死去的妈妈。

I suddenly thought of my dead mother.

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