ずいぶん・随分zuibun[1]atamadaka (head-high)

1. adverb (with と)very; extremely; surprisingly; fairly; quite; considerably; awfully; terribly
   私わたしたちはもうずいぶん長ながくここにいます。↗

   We've been here long enough.

   あなたはこの１年ねんに英語えいごが随分ずいぶん進歩しんぽした。↗

   You've made remarkable progress in English in the past year.
2. na-adjectiveterrible; horrid; contemptible; reprehensible
3. na-adjectiveremarkable; extraordinary

• SynonymなかなかCasualColloquial: 'quite, considerably' with a tone of surprise or admiration, as in 「なかなか上手」; before a negative it means 'not easy'.
• SynonymかなりNeutralNeutral: 'considerably', as in 「かなり疲れた」; more objective than 「随分」 and with less of its surprise.
• SynonymとてもCasualColloquial: 'very', as in 「とても美味しい」; more common than 「随分」 and without its sense of surprise.
• Synonym大変たいへんNeutralNeutral: 'very' or 'seriously', as in 「大変お世話になりました」; common in both speech and writing, and also a na-adjective for 'terrible'.
• SynonymよほどCasualColloquial: 'quite, very', as in 「よほど疲れていた」; implies a degree deep enough that you infer it.
• SynonymよっぽどCasualA colloquial, emphatic form of 「余程」, as in 「よっぽど好きなんだね」; even more casual.
• Synonym甚だはなはだWrittenWritten/formal: 'exceedingly', as in 「甚だ遺憾」; used in formal statements and often negative.
• Synonym極めてきわめてWrittenWritten/formal: 'extremely', as in 「極めて重要」; typical of reports and official documents.
• SynonymたいそうWrittenWritten/formal: 'very' or exaggerated, as in 「大層な口をきく」; carries an old-fashioned tone.
• Synonym実にじつにNeutralNeutral: 'truly, sincerely', as in 「誠にありがとうございます」; standard in formal thanks and apologies; 「随分」 instead carries surprise.
• Synonym非常にひじょうにNeutralNeutral: 'very', as in 「非常に危険」; more formal than 「随分」 and with less subjective emotion.
• SynonymひどくCasualColloquial: 'badly, severely', as in 「ひどく疲れる」; the degree it marks is negative.
• Synonym至っていたってWrittenLeans written: 'extremely', as in 「至って元気」「至って簡単」; said of people and things alike; 「随分」 adds surprise.
• SynonymだいぶNeutralNeutral: 'considerably, by a lot', as in 「だいぶ良くなった」; focuses on the degree of change.
• Synonym大いにおおいにNeutralNeutral: 'greatly', as in 「大いに楽しむ」; mostly modifies verbs.
• Antonym少しすこしNeutralNeutral: the opposite of 「随分」 — 'a little, slightly', as in 「少し寒い」; common in both speech and writing.
• AntonymわずかWrittenWritten/formal: the opposite of 「随分」 — 'only a tiny amount', as in 「僅かな差」.
• AntonymちょっとCasualColloquial: the opposite of 「随分」 — 'a little, slightly', as in 「ちょっと待って」.

Use 「随分」 when 'quite' or 'rather' comes with a feeling of surprise. For plain 'very' or 'quite', reach for 「とても」 or 「かなり」; 「非常に」 and 「極めて」 fit formal prose; 「だいぶ」 tracks degree of change; 「誠に」 suits formal thanks; for 'a little' say 「少し」 or 「ちょっと」.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. はなはだしいさま。相当そうとう。
2. ひどい。あんまり。
3. はなはだしいさま。相当そうとう。非常ひじょうに。かなり。
4. それなりに。
5. （やや古いにしえ）可能かのうな限かぎり。なるたけ。
6. （やや古いにしえ）別わかれの挨拶あいさつの際さいに言いう言葉ことば。くれぐれも。
7. 分相応ぶんそうおうであること。