“〜hazu da・〜hazu ga nai”: Should Be vs Can't Possibly Be (logic-based confidence) [JLPT N3]
Content by NTJ Kuma · Nihongo to Japan editorial team · Updated July 27, 2026
彼は来るはずだ vs そんなことがあるはずがない — logic-based confident expressions, essential at N3.
「約束したから、来るはずだ」 — 「はずだ」, inferring “should be” from reason
「彼女は約束したから、来るはずだ」 (she promised, so she should come) — 「(plain form) ＋ はずだ」 means “should ~, ought to ~,” a conclusion inferred from reason and known facts (not a mere guess, but “I have grounds to think so”). Its negative 「〜はずがない」 = can't be, by reason. This guide covers 「はずだ」's inferential feel and attachment, and separates it from the mere-guess 「かもしれない」 (「はずがない」 has its own in-depth guide).
Core: infer “should be so” from reason
「はずだ」 = “should ~, ought to ~,” inferred from logic and known facts:
- Inferring it should: 10時に出たはずだ (they left at 10, so should be (there)).
- Realization (that's why): 道理で寒いはずだ、雪が降っている (no wonder it's cold — it's snowing).
- Should have but didn't (はずだった): 来るはずだったのに、来なかった (they should have come, but didn't).
Attachment: plain form ＋ はずだ (na-adj 〜な ＋ はず, noun 〜の ＋ はず).
How to Connect
| Form | Use | Example |
|---|---|---|
| verb/i-adj ＋ はずだ | infer it should | 来るはずだ |
| na-adj 〜な ＋ はずだ | na-adjective | 元気なはずだ |
| noun 〜の ＋ はずだ | noun | 休みのはずだ |
| 〜はずだった | should have but… | 来るはずだった （was supposed to come） |
例句（看情境）
- 彼女は約束したから、来るはずだ。(She promised, so she should come.) — inference
- 今日は休みのはずなのに、学校に人がいる。(It should be a day off, yet there are people at school.) — inference (a discrepancy)
- 道理で寒いはずだ、窓が開いていた。(No wonder it's cold — the window was open.) — realization
- 彼は来るはずだったが、急用で来なかった。(He should have come, but couldn't due to an urgent matter.) — should have
- 10時発なら、もう着いているはずだ。(If they left at 10, they should have arrived by now.) — inference
⚠️ Where learners slip up
① 「はずだ」 (grounded inference) vs 「かもしれない」 (mere possibility). はずだ = with reason, ought to be so (約束したから来るはずだ = they promised, so (by reason) should come — high confidence); かもしれない = maybe, possibly (来るかもしれない = might come, just a guess, low confidence). はずだ carries “I have grounds, I'm confident.”
② 「はずだ」 vs 「はずがない」 (opposites). はずだ = should (be); はずがない = can't be, by reason (definitely not). 彼は来るはずだ (should come) ↔ 彼は来るはずがない (can't possibly come). Don't reverse them (はずがない: see its guide).
③ 「〜はずだった」 = should have but didn't (a gap). 来るはずだったのに (should have come, but…) — the past form marks “by reason it should have been so, but it didn't happen,” with surprise/disappointment. A useful extension.
🔄 Compare: はずだ／かもしれない／に違いない
| Form | Confidence／nuance | Example |
|---|---|---|
| 〜はずだ | grounded inference (ought to) | 来るはずだ |
| 〜かもしれない | maybe, possibly (low) | 来るかもしれない （might come） |
| 〜に違いない | must be (high confidence) | 来るに違いない （must be coming） |
| 〜はずがない | can't be, by reason | 来るはずがない （there's no way they'd come） |
💡 Quick takeaway
The core of 「はずだ」 is “inferring ‘it ought to be so' from grounds” — not a random guess (that's かもしれない), but “because…, by reason it should…” (約束したから来るはずだ = she promised, so she should come). Three uses: ① inferring it should (もう着いているはずだ); ② realization “no wonder” (道理で寒いはずだ); ③ should have but didn't (はずだった) (来るはずだったのに = should have come, but…, a gap). Vs 「かもしれない (maybe)」, はずだ is higher confidence, with reason; vs 「はずがない (can't be)」 it's the opposite. For “by reason it should…,” はずだ fits best. The negative 「はずがない」 has its own in-depth guide.
❓ FAQ
Q: How do I tell 「はずだ」 from 「かもしれない」?
By whether there are grounds, and confidence level. 「はずだ」 = inferring “it ought to be so” with reason/grounds (high confidence): 彼女は約束したから、来るはずだ (she promised, so (by reason) she should come — grounded in “she promised”). 「かもしれない」 = maybe, possibly (just a guess, low confidence): 彼女は来るかもしれない (she might come — a guess, could go either way). So 「はずだ」 is “I have grounds to think so, it's most likely,” 「かもしれない」 is “uncertain, maybe.” With clear grounds use はずだ; for a mere guess use かもしれない.
Q: What does 「はずだった」 mean?
「〜はずだった」 = “(by reason) should have…, but it didn't (happen),” the past form marking “an expectation unmet,” with surprise or disappointment. E.g. 電車は10時に来るはずだったが、遅れた (the train should have come at 10, but was late), 合格するはずだったのに、落ちた (I should have passed, but failed). It differs from plain 「はずだ (should)」: はずだ is a present inference (should…), はずだった is a retrospective “I thought it would…, but it didn't” (a gap). Often paired with 「〜のに (but/yet)」 to heighten the surprise/regret.
🖊️ Practice Questions (5)
Q1. 「彼女は約束したから、来る___。」 (confidence based on a promise) Which fits the blank?
(A) かもしれない (B) でしょう (C) はずだ (D) らしい
Q2. 「あんなに頑張ったのに、失敗する___！」 (there's no way she fails!) Which fits the blank?
(A) はずだ (B) はずがない (C) に違いない (D) かもしれない
Q3. 「彼は来る___、来なかった。」 (he was supposed to come but didn't) Which fits the blank?
(A) はずなのに (B) はずだったのに (C) はずがないのに (D) はずかもしれないが
Q4. はずだとかもしれないの最大の違いは？
(A) はずだ is the past, かもしれない is the future
(B) はずだ is confidence with a logical basis; かもしれない is a possibility with weak grounds
(C) はずだ is negative, かもしれない is affirmative
(D) exactly the same, interchangeable
Q5. 「今日は休みな___、学校は閉まっている。」 (attaching はずだ to a noun) Which fits the blank?
(A) のはずだから (B) だはずから (C) はずから (D) のはずがあるから
Answer Explanations
1. (C) はずだ ── 「約束した (there's a basis)」→ はずだ (logical confidence). かもしれない is too uncertain; でしょう has a weaker basis.
2. (B) はずがない ── a strong denial of possibility: 「失敗するはずがない」 = there's no way she fails.
3. (B) はずだったのに ── はずだった = was supposed to but didn't (expectation unfulfilled). 「来るはずだったのに、来なかった」.
4. (B) ── はずだ is confidence with a logical basis (a promise, a rule, common sense); かもしれない has weak grounds, just a possibility.
5. (A) のはずだから ── attaching はずだ to a noun: noun + のはずだ. 「休みのはずだから」.