Both 突然 (tūrán) and 忽然 (hūrán) express the something happens in a very short time or is unexpected. Both can be translated as "unexpectedly" or "surprisingly," however, 突然 is a bit harsher in tone while 忽然 is a bit softer. Also, their grammatical features are subtly different, as outlined in this article.
Both 突然 and 忽然 can be used as Adverbs
突然 and 忽然 both can put in the sentence as adverbs or adjectives, just make sure to put them before the verb or the adjective!
Before a Verb
Examples
- 睡觉 的 时候，我 的 电话突然 响了。 While I was sleeping, my phone suddenly started to ring.
- 睡觉 的 时候，我 的 电话忽然 响了。 While I was sleeping, my phone suddenly started to ring.
- 他 说着说着突然 不 说 了。 He was talking and talking, and suddenly stopped.
- 他 说着说着忽然 不 说 了。 He was talking and talking, and suddenly stopped.
Before an Adjective
Examples
- 天 突然 暗 了。
- 天 忽然 暗 了。 The sky suddenly got dark.
- 灯 突然 不 亮 了。
- 灯 忽然 不 亮 了。 The lights suddenly went out
At the Beginning of a Sentence
You can place 突然 and 忽然 at the beginning of a sentence, just make sure to add a comma after it!
Structure
Examples
- 突然，他 进来 了。
- 忽然，他 进来 了。 Suddenly, he entered.
突然 can be used as an Adjective
突然 can be used as an adjective (directly modifying nouns), while 忽然 cannot.
Structure as an Adjective
Examples
- 这 是 一 场 突然 事故，我们 都 没 想到。 This was a sudden accident. None of us thought it could happen.
- 你 知道 怎么 处理 这 种 突然 的 情况 吗？ Do you know how to handle this kind of sudden situation?
突然 can be used as a complement
突然 as an adjective can also be used as a complement (called 补语 in Chinese). 忽然 cannot be used as a complement.
Structure for verb and complement
Examples
- 事情 发生 得 太 突然，我们 不 知道 怎么 办。 Things happened so suddenly, we didn't know what to do.
- 这 场 雨 下 得 太 突然，都 淋湿 了。 This rain fell so suddenly, everything got soaked.
突然 can be used as the predicate
Just like other adjectives, 突然 can be used as the predicate of a sentence. It is usually added with a modifier like 很, 非常, 不 etc. It can also end with 得很, 得多, 极了, etc. complements.
Structure with modifier + 突然
Examples
- 这 个 决定 太 突然 了。 This decision was too sudden.
- 对 她 来说，这 次 升职 其实 不 突然。 To her, this promotion wasn't actually surprising.
Structure with 突然 + modifier
Examples
- 离开 上海 突然 得 很。 Leaving Shanghai is really surprising.
- 这 次 生病 比 上 次 突然 得 多。 Getting sick this time is much more surprising than the last time.
Example of right and wrong sentences
- 这 个 决定 很 忽然。
- 这 个 决定 很 突然。 突然 can be a predicate, while 忽然 cannot.
- 一切 发生 得 太 忽然。
- 一切 发生 得 太 突然。 突然 can be a complement, but 忽然 cannot.
Example Dialog
- A: 我 正在 睡觉，忽然 电话响了，是 Amy 打 来 的。 I was sleeping, when suddenly the phone rang. It was Amy.
- B: 她 说 什么？ What did she say?
- A: 她 结婚 了。 She got married.
- B: 啊，这 个 消息 太突然 了。 Oh, this news is really sudden.
Sources and further reading
Books
- Integrated Chinese: Level 2, Part 2 (pp. 231-2)
- Chinese Grammar - Broken down into 100 items - Basic and Intermediate Levels ()(突然 忽然p.113)
Dictionaries
- 现代汉语词典（第5版） (忽然p.574 突然p.1378)