大変・大へん|たいへんtaihen[0]heiban (flat)

1. adverbvery; greatly; terribly; awfully
   １日にち中ちゅう歩き回あるきまわって大変たいへん疲つかれた。↗

   I am dead tired from walking around all day.
2. na-adjective, nounimmense; enormous; great
   あえて言いいますが、もし我々われわれがそんなことをすれば大変たいへんな誤あやまりを犯おかすことになると私は思おもいます。↗

   At the risk of sticking my neck out, I think that if we do that, we're making a big, big mistake.
3. na-adjective, nounserious; grave; dreadful; terrible
   もし期限切きげんぎれになったら、大変たいへんなことになるよ。↗

   There will be hell to pay if I don't make this deadline.
4. na-adjective, noundifficult; hard; challenging
5. nounmajor incident; disaster

• Synonym誠にまことにFormalFormal thanks and apologies: 「誠にありがとうございます」. Adverb only, while 「大変」 also works as a na-adjective meaning tough going.
• SynonymずいぶんCasualColloquial, often with a note of surprise that things exceed expectation. In formal speech it survives in set phrases like 「随分ご無沙汰しております」.
• SynonymごくNeutralSeen in 「ごく一部」 and 「ごく普通」: stresses an extreme degree or a very narrow range. It has no 'tough going' sense, unlike 「大変」.
• Synonym思い切りおもいきりCasualMeans doing something to the full, with all one's strength; it modifies the force of an action. It overlaps with 「大変」 only as an intensifier.
• Synonym甚だはなはだWrittenStrongly written in flavor and often carries a negative judgment, as in 「甚だ遺憾」. Rare in everyday conversation.
• Synonym極めてきわめてWrittenA written, objective 'extremely', at home in reports and papers (「極めて重要」); you rarely hear it in casual chat.
• Synonym非常にひじょうにNeutralA neutral, objective 'very', more common in writing. For formal spoken thanks or apologies, 「大変」 is the usual choice.
• SynonymひどくNeutralUsually negative (「ひどく疲れる」, 「ひどく叱られる」) and stresses that the degree is excessive.
• SynonymたいそうWrittenA slightly dated 'very' that also covers exaggeration or making a fuss of things (「大層なことを言う」).
• Synonym実にじつにFormalThe same word as 「誠に」 above. Used in formal thanks and apologies; adverb only, while 「大変」 also serves as a na-adjective meaning tough going.
• Synonym至極しごくWrittenOften follows a kanji compound (「残念至極」) and also works as an adverb (「至極当然」). Stiff and written in tone.
• Synonym大いにおおいにNeutralMostly modifies verbs (「大いに期待する」, 「大いに反省する」); with adjectives it is less natural than 「大変」.
• Synonym至っていたってWrittenSlightly written; mostly modifies states (「いたって元気」, 「いたって普通」) with a calm, matter-of-fact tone.
• SynonymいかにもNeutralStresses 'indeed, just as one would expect' — an assertion or exclamation (「いかにも彼らしい」) — rather than plain degree.
• SynonymとびきりCasualMarks something as outstanding within its kind (「とびきりの笑顔」); mostly used in positive evaluations.
• SynonymなかなかCasualA 'considerably' that exceeds expectation, often with a tone of praise. With a negative it means 'not easily…'.
• SynonymよほどNeutralOften used to guess at a degree (「よほど疲れていたのだろう」) or in comparisons (「よほどましだ」).
• SynonymとてもCasualThe everyday workhorse 'very', colloquial in feel; with a negative it means 'not at all, no matter what'. 「大変」 is the more formal pick.
• Synonym何ともなんともNeutralOften voices a feeling hard to put into words (「何とも言えない」); with a negative it means 'not at all'.
• SynonymよっぽどCasualA colloquial, emphatic form of 「余程」 with the same meaning, only more casual.
• SynonymごくごくCasual「極」 doubled for emphasis; typically modifies words like 「普通」 and 「一部」. Leans colloquial.
• SynonymいたくWrittenMostly modifies psychological verbs such as being moved or impressed (「いたく感動する」). Slightly literary and not often used.
• SynonymすこぶるWrittenA slightly old-fashioned 'very', scarce today; found mainly in writing or with a teasing tone.
• SynonymいともWrittenUsually appears in set collocations like 「いとも簡単に」 or 「いともたやすく」; written in flavor and seldom used on its own.
• Related最ももっともNeutralThe superlative 'most' within a comparison (「最も高い」), not a plain 'very' like 「大変」.
• Related最さいWrittenMainly a prefix (「最先端」, 「最優秀」) meaning 'most'; it cannot stand alone as an adverb the way 「大変」 can.
• Antonym簡単かんたんNeutralThe opposite of 「大変」 in its 'effortful, tough' sense: simple and easy to do.
• Antonym楽らくNeutralThe opposite of 「大変」 in its 'hard going' sense: comfortable and free of effort.

To say 'very': 「とても」 is the everyday word, 「大変」 and 「誠に」 the formal ones, and 「非常に」 or 「極めて」 the written ones. The rest are narrower — superlatives (「最も」, 「最」), negative or evaluative shades (「酷く」, 「中々」), verb-focused (「大いに」) and more. For the 'tough going' sense, the opposites are 「楽」 and 「簡単」.

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

1. 重大じゅうだいな事件じけん。
2. 物事ものごとが深刻しんこくであること。大おおいに困こまること。
3. 苦労くろうなどが甚はなはだしいこと。実現じつげんすることや対処たいしょすることが困難こんなんなこと。
4. 程度ていどがはなはだしいこと。
5. 非常ひじょうに。大層たいそう。