“osoraku・tabun・kitto・moshikashitara”: Conjecture Adverbs (degrees of certainty) [JLPT N3]

Content by NTJ Kuma · Nihongo to Japan editorial team · Updated July 27, 2026

Comparing conjecture adverbs from high to low certainty.

きっと? たぶん? もしかしたら? — the “certainty ladder” of guessing adverbs

For the same “it'll probably rain,” きっと (surely), おそらく (probably), たぶん (probably), もしかしたら (maybe) differ a lot in tone — Japanese guessing adverbs line up on a ladder from high to low certainty, each paired with a matching sentence ending. This guide teaches how to pick the right guessing adverb ＋ ending by “how sure you are,” and separates them from 「必ず (certainly, without exception)」.

Core: pick the right guessing adverb ＋ matching ending by “how sure you are”

Guessing adverbs are chosen by level of certainty, each with a matching ending:

• きっと (surely) = strong subjective certainty/expectation, with 「だろう／にちがいない」: きっと合格するだろう.
• おそらく (probably) = higher certainty, more written, with 「だろう」: おそらく問題ないだろう.
• たぶん (probably) = general guess, colloquial, with 「だろう／と思う」: たぶん来るだろう.
• もしかしたら (maybe) = lowest certainty, with 「かもしれない」: もしかしたら雨が降るかもしれない.

Certainty & Ending Chart

Examples in context

• あんなに勉強したんだから、きっと合格するよ。(You studied so much — you'll surely pass.) — きっと (certainty)
• おそらく、彼の判断は正しいだろう。(His judgment is probably right.) — おそらく (written)
• たぶん、明日は晴れると思う。(It'll probably be sunny tomorrow, I think.) — たぶん (colloquial)
• もしかしたら、雨が降るかもしれない。(Maybe it'll rain.) — もしかしたら (low)
• 彼はきっと来るにちがいない。(He'll surely come.) — きっと ＋ にちがいない

⚠️ Where learners slip up

① The adverb and ending must agree — don't mismatch. もしかしたら pairs with かもしれない (もしかしたら雨かもしれない), きっと with だろう／にちがいない (きっと来るだろう). ✗ もしかしたら来るにちがいない (low certainty ＋ high assertion — contradictory), ✗ きっと来るかもしれない (high certainty ＋ low guess — odd). The adverb's certainty must match the ending's.

② 「きっと」 (subjective certainty) vs 「必ず」 (necessity, 100%). きっと = strong subjective certainty/expectation (‘I believe it'll surely,' yet still a guess) (きっと合格する = will surely pass, with expectation); 必ず = objective necessity, 100% (‘without exception,' or strong will) (必ず提出してください = be sure to submit, 水は必ず低いほうへ流れる = water always flows downward). きっと is subjective, with expectation; 必ず is objective necessity or compulsion. For “(I believe) surely” use きっと; for “(without exception/be sure to) certainly” use 必ず.

③ 「おそらく」 is more written, 「たぶん」 more colloquial. Their certainty is close (both “probably, very likely”), differing in register: おそらく = more written, formal (reporting, discourse); たぶん = colloquial, everyday (common in conversation). Same meaning — use おそらく in writing, たぶん in chat.

🔄 Compare: きっと／おそらく／たぶん／もしかしたら／必ず

💡 The one thing to remember

Japanese guessing adverbs are like a “certainty ladder,” high to low: きっと (surely, subjective certainty/expectation) → おそらく (probably, written) → たぶん (probably, colloquial) → もしかしたら (maybe, lowest). The key is matching the adverb to an ending of matching certainty: きっと with だろう/にちがいない, たぶん with だろう/と思う, もしかしたら with かもしれない — don't pair the low-certainty もしかしたら with the high-assertion にちがいない. Also don't mix 「きっと」 and 「必ず」: きっと = subjective ‘I believe it'll surely' (still a guess, with expectation); 必ず = objective ‘without exception' or ‘be sure to' (necessity, compulsion). To accurately express “how sure you are,” first think “how confident am I,” then pick the matching adverb and ending.

❓ Quick FAQ

Q: How do I tell 「きっと」 from 「必ず」? (both “certainly”)

By “subjective certainty/expectation” vs “objective necessity/compulsion.” 「きっと」 = strong subjective certainty, expectation (‘I believe it'll surely …,' still fundamentally a guess): 明日はきっと晴れるよ (it'll surely be sunny tomorrow — my expectation/belief, though the weather is actually uncertain), 彼ならきっと成功する (he'll surely succeed — I believe so), きっと大丈夫だよ (it'll surely be fine — reassurance, expectation). It carries the speaker's subjective feeling (belief, expectation), often paired with 「だろう／にちがいない」. 「必ず」 = objective necessity (without exception) or strong will/requirement (be sure to): 水は必ず低いところへ流れる (water always flows to low ground — an objective law, 100%), 宿題は必ず提出してください (be sure to submit the homework — a compulsory requirement), 必ず戻ってくる (I'll definitely come back — strong will). It carries no “guess,” but “necessity” or “must.” So: “(I believe/expect) surely” → きっと (subjective, a guess); “(without exception/be sure to) certainly” → 必ず (objective necessity/compulsion). For “uncertain but I believe” things like weather or whether someone will come, use きっと; for laws, rules, firm resolve — “absolute, without fail” — use 必ず.

Q: Why must guessing-adverb endings ‘agree'? How do I pair them correctly?

Because in Japanese the adverb's certainty and the ending's certainty must match (agree front to back); mismatching sounds unnatural. Memorize by pairs: ① きっと (high certainty) → だろう／にちがいない (きっと来るだろう／きっと来るにちがいない = will surely come); ② おそらく／たぶん (mid-high) → だろう／と思う (たぶん来るだろう = will probably come); ③ もしかしたら (low) → かもしれない (もしかしたら来るかもしれない = might come). Wrong examples: ✗ もしかしたら来るにちがいない (“maybe” is low certainty, “にちがいない” is high assertion — low front, high back, contradictory), ✗ きっと来るかもしれない (“surely” with “maybe” — high front, low back, also odd). Mnemonic: a high-certainty adverb pairs with a high-certainty ending (きっと ＋ だろう/にちがいない), a low one with a low one (もしかしたら ＋ かもしれない). First settle “how sure I am,” pick front and back together, and you won't mismatch.

🖊️ Practice Questions (5)

Q1. 「___雨が降るかもしれないから、傘を持って行こう。」(it might rain)

(A) もしかしたら　(B) 必ず　(C) きっと　(D) ぜひ

Q2. 「あんなに勉強したんだから、___合格するよ。」(surely will pass — subjective conviction)

(A) きっと　(B) もしかしたら　(C) たぶん　(D) わざと

Q3. Which has the lowest degree of certainty?

(A) もしかしたら

(B) きっと

(C) おそらく

(D) たぶん

Q4. Which sentence-ending pairs naturally with 「もしかしたら」?

(A) 〜かもしれない

(B) 〜てください

(C) 〜なさい

(D) 〜だ (assertion)

Q5. Which correctly states the difference between 「きっと」 and 「必ず」?

(A) きっと = subjective conviction/expectation; 必ず = objective certainty (100%)

(B) They are exactly the same

(C) きっと = objective 100%

(D) 必ず = maybe

Answer Explanations

1. (A) もしかしたら ── Pairs with かもしれない, "maybe".

2. (A) きっと ── Strong subjective conviction.

3. (A) もしかしたら ── Lowest certainty.

4. (A) 〜かもしれない ── The pairing for もしかしたら.

5. (A) ── きっと subjective; 必ず objective 100%.