English gets away with a handful of overworked modals — maybe, probably, should, must — and lets your tone of voice fill in how sure you really are. "He must be coming" and "he might be coming" share almost all their machinery; only must versus might carries the weight. Japanese does the opposite: it gives each degree of confidence its own word, so the choice is forced and visible. Pick the wrong rung and you don't just sound slightly off — you over-commit or under-commit in a way a native immediately hears. This page lines the rungs up so you can choose deliberately.
Two questions organize the whole field. First, how sure are you? — the degree axis. Second, why do you believe it? — the basis axis (did you see evidence, reason it out, feel convinced, or hear it from someone?). Nail both and you'll never again translate "probably" by reflex.
The degree axis: the confidence ladder
From least to most committed:
| Form | Confidence | English feel | Basis |
|---|---|---|---|
| 〜かもしれない | low — real doubt | "might, maybe" | open possibility |
| 〜でしょう / だろう | medium — confident guess | "probably, I'd say" | general expectation |
| 〜はず | high — logical expectation | "supposed to, ought to" | objective reasoning |
| 〜にちがいない | very high — conviction | "surely, must be" | subjective certainty |
The cleanest way to feel the ladder is to hold the proposition fixed and climb it. Take 彼は来る ("he's coming") and change only the modal:
彼は来るかもしれない。
kare wa kuru kamoshirenai
He might come. (you genuinely don't know)
彼は来るでしょう。
kare wa kuru deshō
He'll probably come. (a confident guess)
彼は来るはずだ。
kare wa kuru hazu da
He's supposed to come. (the plan/facts say so)
彼は来るにちがいない。
kare wa kuru ni chigainai
He must be coming — I'm sure of it. (personal conviction)
Read top to bottom, your commitment rises from a shrug to near-certainty. Notice that only the words changed; the event never did. That is the spectrum in one breath.
The basis axis: why do you believe it?
Degree isn't the whole story. Two sentences can be equally sure yet built on different grounds, and Japanese marks that too. This is where the "seems" family earns its place.
| Basis | Form | Gloss |
|---|---|---|
| Direct evidence you see/sense (imminent) | 〜そう (stem + そう) | "looks like it's about to…" |
| Inference from observation | 〜よう / 〜みたい | "it seems / appears…" |
| Indirect signs or things heard around | 〜らしい | "apparently, seems…" |
| Reported speech / hearsay | 〜そうだ (plain + そうだ) | "I hear that…" |
| Logical deduction from facts | 〜はず | "ought to, supposed to" |
| Inner conviction | 〜にちがいない | "must be, surely" |
Watch a single situation — it's going to rain — get told four ways, each flagging a different source:
空が暗くなってきた。雨が降りそうだ。
sora ga kuraku natte kita. ame ga furisō da
The sky's darkening — it looks like it's about to rain. (I can see it coming)
天気予報によると、明日は雨が降るらしい。
tenki yohō ni yoru to, ashita wa ame ga furu rashii
According to the forecast, it's apparently going to rain tomorrow. (secondhand info)
予報でそう言っていたから、午後は雨が降るはずだ。
yohō de sō itte ita kara, gogo wa ame ga furu hazu da
The forecast said so, so it should rain in the afternoon. (reasoned conclusion)
こんなに湿気がひどいんだから、ひと雨来るにちがいない。
konna ni shikke ga hidoi n da kara, hito-ame kuru ni chigainai
With the humidity this bad, a shower must be coming. (I'm convinced)
One warning the basis axis makes visible: そう splits in two depending on what it attaches to, and the difference is entirely about basis. Stem + そう (降りそう) reports what you can see happening — imminent, visual. Plain form + そうだ (降るそうだ) reports hearsay — what you were told. Same syllables, opposite sources:
空を見て。今にも降りそうだよ。
sora o mite. ima ni mo furisō da yo
Look at the sky — it's about to pour any minute. (I can see it → appearance そう)
天気予報によると、明日は雨が降るそうだ。
tenki yohō ni yoru to, ashita wa ame ga furu sō da
According to the forecast, I hear it's going to rain tomorrow. (reported → hearsay そうだ)
The "seems" trio — そう, よう/みたい, らしい — is a whole sub-system with its own subtleties (visual immediacy vs. reasoned inference vs. distance from the source). Untangling them is the job of そう・よう・みたい・らしい compared; here, just file them under evidence-flavored guessing and remember that they answer why, while the four-rung ladder answers how sure.
The trap: "must" is not "might," and not "probably"
The single most damaging spectrum error for English speakers is rendering a confident deduction with a doubtful form. English "he must be coming" is high on the ladder — it is a firm inference. Learners map its surface onto かもしれない ("might") or soften it to でしょう ("probably"), and the result quietly leaks doubt the speaker never intended.
❌ 電気がついている。彼は家にいるかもしれない。
denki ga tsuite iru. kare wa ie ni iru kamoshirenai
Undershoots — the lights being on is evidence; 'might' throws away your deduction.
✅ 電気がついているから、彼は家にいるはずだ。
denki ga tsuite iru kara, kare wa ie ni iru hazu da
The lights are on, so he ought to be home. (reasoned)
✅ 電気がついている。彼は家にいるにちがいない。
denki ga tsuite iru. kare wa ie ni iru ni chigainai
The lights are on — he must be home. (conviction)
Both はず and にちがいない are legitimate here; かもしれない is not, because you have evidence. The rule of thumb: reserve かもしれない for when you genuinely could be wrong, でしょう for a confident lean, and climb to はず / にちがいない the moment you have real grounds.
A quick decision path
When you catch yourself about to say "probably/should/must," run this:
- Do I have specific grounds (a schedule, a fact, a promise)? → はず.
- Am I inwardly certain, grounds or not? → にちがいない.
- Do I just lean toward yes, no special evidence? → でしょう / だろう.
- Could I honestly be wrong? → かもしれない.
- Am I reporting appearances or what I heard, not asserting? → そう / よう / らしい / そうだ.
約束したんだから、彼女は時間どおりに来るはずだ。
yakusoku shita n da kara, kanojo wa jikan-dōri ni kuru hazu da
She promised, so she ought to arrive on time. (grounds → はず)
この味、どこかで食べたことがあるかもしれない。
kono aji, doko ka de tabeta koto ga aru kamoshirenai
This taste — I might have had it somewhere before. (honest uncertainty → かもしれない)
Common Mistakes
❌ 犯人は彼でしょう。証拠が全部そろっている。
hannin wa kare deshō. shōko ga zenbu sorotte iru
Too weak — with all the evidence in, 'probably' undersells it.
✅ 犯人は彼にちがいない。証拠が全部そろっている。
hannin wa kare ni chigainai. shōko ga zenbu sorotte iru
He must be the culprit — all the evidence lines up.
❌ たぶん彼は来るにちがいない。
tabun kare wa kuru ni chigainai
Contradictory — 'maybe' (たぶん hedge) clashes with the conviction of にちがいない.
✅ きっと彼は来るにちがいない。
kitto kare wa kuru ni chigainai
He'll surely come — I'm certain of it. (きっと matches the conviction)
❌ 見て、彼は今にも泣くだろう。
mite, kare wa ima ni mo naku darō
Off — when the sign is right in front of you, だろう ('I'd guess') ignores the visible evidence.
✅ 見て、彼は今にも泣きそうだ。
mite, kare wa ima ni mo nakisō da
Look, he's about to cry any second. (visible evidence → そう)
Key Takeaways
- Japanese assigns each degree of confidence its own word: かもしれない < でしょう/だろう < はず < にちがいない.
- Choose on two axes: how sure (the ladder) and why (evidence → そう/よう/らしい; logic → はず; conviction → にちがいない; report → らしい/そうだ).
- English "must" is high on the ladder — render it with はず or にちがいない, never with かもしれない or a bare でしょう.
- Match your adverbs to the rung: たぶん pairs with でしょう; きっと/絶対 pair with にちがいない.
- Treat these forms as one ordered system, not isolated points — that is the whole reason to study them together.
Now practice Japanese
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Start learning Japanese→Related Topics
- 〜かもしれない: Possibility ('might')N3 — 〜かもしれない ('might / maybe / perhaps') — the genuine-doubt, roughly-coin-flip end of the conjecture scale — how it attaches to plain forms and bare nouns, its casual clip 〜かも, and why it sits far below でしょう, はず and にちがいない in confidence.
- 〜はず: Expectation ('supposed to')N3 — How Japanese states a logical expectation drawn from known facts — 来るはずだ 'should be coming' — plus はずがない ('couldn't possibly') and the regretful はずだった ('was supposed to, but…').
- 〜にちがいない: Conviction ('must be')N2 — 〜にちがいない ('must be / surely / no doubt') — the top of the certainty scale, expressing the speaker's confident deduction from evidence — how it attaches, why it's the inferential 'must' and never the obligation 'must', and how it differs from objective はず.
- でしょう / だろう: ConjectureN4 — でしょう/だろう as the everyday 'probably' of the modality system — a confident guess that sits above かもしれない and below はず, and, with a rising tone, doubles as the tag 'isn't it?'
- そう / よう / みたい / らしい ComparedN3 — The decision page for the four Japanese ways to say 'seems / looks / apparently' — 〜そう (direct perception), 〜ようだ and 〜みたいだ (your own reasoning, formal vs casual), and 〜らしい (secondhand report) — chosen by evidence source and register, not by English wording.