The differences between いきなり, [急]{きゅう}に and [突然]{とつぜん}

Question

I've come to see [突然]{とつぜん} as "sudden in an almost overwhelming manner". The other two, however, I am unable to figure out. What are the differences between these words, and when is one more appropiate than the other?

Answer 1

I'm not sure I agree with the formality distinctions since I hear all of them used in a variety of situations, but that is neither here nor there. As I don't have a true native's insight I will cite some materials produced by natives on the subject.

This question is succinctly answered in this alc entry. To summarize, it says the following:

• They all mean roughly the same thing, and in the sentence 「子どもが○○飛び出してきた」 any could fill the blank and be acceptable.
• In the sentence "第一問からいきなり詰まってしまった。" いきなり cannot be replaced with either of the other two. There is a nuance here of beginning an action without proper precaution, of starting something without the expected timing or process. The others do not have this nuance.
• For the other two, consider the sentence バスが突然急に停まった。 急に describes the way in which an action is carried out. 突然 on the other hand can be thought of describing what's going on (出来事の生じる様子 as the article states it).

That difference is a little abstract and hard to understand though, so I will cite some examples from the explanation written here:

These sentences can naturally use both:

例１１）急に／突然　雨が降ってきて、あわてて建物の中に入った。
例１２）さっきまで楽しく歌っていたと思ったら、急に／突然　泣き出した。

The following feel more natural with 急に rather than 突然:

例１３）花子は中学に入って、急に背が伸びた。
例１４）彼がコーチになって、あのチームは急に強くなった。

The explanation provided is that 急に represents something that changes in a short time while 突然 is something unexpected that happens quickly. In 13 and 14 突然 could be used, but since getting taller or a team getting stronger are more of a process of change (変化の過程), 急に is better. To quote fully, they are summarized as this:

「急に」：状況／事態の変化の仕方に焦点を当てる
※変化の過程も、一瞬の事態の成立にも使える
「突然」：予想外の事態の成立に焦点を当てる
※一瞬の事態の成立にのみ使える

And for my last copy/paste job, one more example:

例１５）娘が（急に／突然）病気になった。
「急に～」：短時間のうちに、健康な状態から病気の状態に大きく変化したことを伝える。
「突然～」：何も前触れがなく、病気になってしまったことを伝える

So your daughter suddenly got sick. With 急に it means that she went from a healthful condition to an unhealthy condition in a short period. With 突然 it's like saying she suddenly fell ill as if out of nowhere.

I'm not exactly sure how they want to reconcile this idea of a process of change with the acceptability of the sentence "急に飛び出した." It could be that there is generally an overlap but in certain contexts these nuances kick in. Dictionaries don't provide much help since they all have each other in their definitions. Ultimately they're all basically the same and you probably don't need to worry too much about looking dumb if you make a mistake or something.

Answer 2

いきなり is casual and very colloquial.

突然 is more formal, seen in writings often but still fine for colloquial use. Same as いきなり in meaning.

唐突 is the same as 突然 but used with human behavior, not inanimate objects, trains, astroids, etc. Example: 唐突なカミングアウト (unexpected and sudden coming out of a person about something).

急に is focused on sudden in meaning, nothing to do about surprise.

Answer 3

One difference is in place of usage. As you may know, a typical pattern (with certainly plenty of uncorrelated exceptions) is for "natural" Japanese words like いきなり to be favored in oral/informal contexts, whereas imported 漢語 appears more often in written/authoritative contexts. If you use 漢語 all the time in your speech, your friends may tell you you sound like a book ;)

Examples (from "hard" written language to spoken):

• 現在 -> 今
• 柔軟性の高い -> 柔らかい
• 不得意 -> 苦手 (here it matters that にがて is pronounced with 訓読み)
• 流暢 -> ぺらぺら
• 迅速 -> 速い
• 遭遇する -> 遭ってしまう
• 到着する -> 着く
• 運搬する -> 運ぶ
• 切断する -> 切る
• 驚愕 -> 驚き
• 教育方法 -> 教え方
• 構成 -> 成り立ち
• 翌日・明日《みょうにち》 -> 明日《あした》

The above all mean something slightly different, but can replace each other in most cases, changing the tone.

With that in mind, 「いきなりなんなのよ！」　「急に寒くなったね」 (oral) and 突然、雨が止《や》んだ。(written) would I think be representative uses for each of these words.

As for "sudden in an almost overwhelming manner", I think you can get that from any of these three.