In short

〜はずだ (hazu da) is not a feeling - it is a conclusion you are entitled to draw from facts you already have: a timetable, a promise, a rule. 〜に違いない (ni chigainai) is one step stronger, “it can’t be anything else”. The わけ family is about the reasoning itself: 〜わけだ (wake da) says so that’s why, 〜わけがない (wake ga nai) says there’s no way, 〜わけではない (wake de wa nai) says it’s not that…, and 〜わけにはいかない (wake ni wa ikanai) says I can’t allow myself to.

The rule

Step 1 - はずだ: the expected result of something you know.

Plain form + はずだ - and, because はず is a formal noun (“expectation”), a noun links with の and a な-adjective keeps its な, exactly like よう in lesson 49. The verb inside stays plain even when the sentence ends politely: ×着きますはずです -> 三時に着くはずです (San-ji ni tsuku hazu desu, he should arrive at three).

The grounding matters. You say はずです because there is a timetable, a rule, an arrangement or a piece of arithmetic behind you. Take that away and the sentence sounds odd: a bare hunch is でしょう or かもしれません (lesson 32), not はずです.

Step 2 - Two negatives, and they are not the same.

• 〜ないはずだ nai hazu da - “it is expected not to happen”: 今日は来ないはずです (Kyō wa konai hazu desu, he isn’t due to come today).
• 〜はずがない hazu ga nai - “there is no expectation at all”, i.e. impossible given what I know: そんなはずがありません (Sonna hazu ga arimasen, that can’t be right).

Step 3 - はずだった: the expectation that did not hold.

Put はず in the past and you get an expectation the world failed to honour - almost always followed by a “but”: 来るはずだったんですが、来ませんでした (Kuru hazu datta n desu ga, kimasen deshita, he was supposed to come, but he didn’t). The variant 〜はずではなかった hazu de wa nakatta laments the whole situation: こんなはずではありませんでした (Konna hazu de wa arimasen deshita, this wasn’t how it was supposed to turn out).

Step 4 - に違いない and に決まっている: conviction.

Both attach to the plain form, and nouns and な-adjectives attach bare - no だ (×日本人だに違いない).

• 〜に違いない ni chigainai, polite 〜に違いありません - “must be, I’m certain”. Reasoned, usable in writing.
• 〜に決まっている ni kimatte iru, polite 〜に決まっています - literally “it is decided that…”, so: “obviously, of course”. Emotional and colloquial; it dismisses the alternative rather than weighing it. Keep it out of reports and out of conversations with a superior.

Step 5 - わけ means “reason, logic” - and that explains the whole family.

わけ (訳) is a formal noun meaning reason. Every member below is a statement about a line of reasoning, and they are confused constantly, so learn them as a set.

• 〜わけだ - so that’s why / it follows. You have just received a fact and you announce the conclusion it explains. The adverb 道理で (dōri de, no wonder) or つまり (tsumari, in other words) usually leads: 日本に一年も住んでいたんですか。道理で日本語が上手なわけです。
• 〜わけがない - there’s no way. A flat logical impossibility, close to はずがない but more emotional and more absolute.
• 〜わけではない, casual 〜わけじゃない - partial denial. It does not deny the fact; it denies a conclusion the listener might be drawing: 嫌いなわけではありません = it’s not that I dislike it (I have some other reason). This is the single most useful member for real conversation, because it lets you decline without insulting anyone.
• 〜わけにはいかない - I can’t, where the obstacle is social, moral or practical, never ability. See Step 6.

Attachment follows はず for verbs, い-adjectives and な-adjectives (嫌いなわけです), with one exception: before わけ a noun does not take の - it links with な or である (〜なわけです, 〜であるわけです), and very often through という (〜というわけです). わけにはいかない is the odd one out - it attaches to a verb only, never to a noun or an adjective.

Step 6 - わけにはいかない is not the potential form.

飲めません (nomemasen) means your body or the situation makes drinking impossible. 飲むわけにはいきません (nomu wake ni wa ikimasen) means you are perfectly able to drink but must not - you drove, you promised, it would be rude. English “I can’t” covers both; Japanese does not.

Its negative, 〜ないわけにはいかない (nai wake ni wa ikanai), is a double negative meaning I have no choice but to: 行かないわけにはいきません (Ikanai wake ni wa ikimasen, I really have to go).

Step 7 - 〜とは限らない and 〜と言っても: trimming an overstatement.

〜とは限らない to wa kagiranai (not necessarily) attacks a generalization, not a fact: 高い店がいつもおいしいとは限りません。 〜と言っても to itte mo (although I say…, it’s not much) pre-empts the listener’s exaggerated picture: 日本語ができると言っても、簡単な会話だけです (Nihongo ga dekiru to itte mo, kantan na kaiwa dake desu, I say I speak Japanese, but only simple conversation).

What this lesson is not. English “should” also means advice - that is 〜たほうがいいです in lesson 35, never はずです. Evidence-based “seems” (ようだ, らしい) is lesson 49. To state an opinion softly rather than firmly, use 〜のではないでしょうか (no de wa nai deshō ka) from lesson 32 - it is the polite opposite end of everything on this page.

Forms & patterns

Attachment - はず takes の／な like a noun; に違いない takes nouns bare:

The certainty scale, with its negatives underneath:

〜かもしれない and 〜でしょう belong to lesson 32; the bottom three are this lesson. Note that 〜とは限らない sits outside the scale - it denies a generalization, not a fact.

The わけ family - the contrast set:

Four ways to say “I can’t drink” - only one is about ability:

Examples

Common mistakes

• 三時に着きますはずです。 (San-ji ni tsukimasu hazu desu.) -> 三時に着くはずです。 (San-ji ni tsuku hazu desu.) - はず is a noun, so the clause in front of it is a noun-modifying clause and the verb must be plain. The politeness is carried once, at the end.
• 彼は学生はずです。 (Kare wa gakusei hazu desu.) -> 彼は学生のはずです。 (Kare wa gakusei no hazu desu.) - a noun links to はず with の, and a な-adjective keeps な: 上手なはずです, not ×上手はずです.
• 彼は日本人だに違いありません。 (Kare wa Nihonjin da ni chigai arimasen.) -> 彼は日本人に違いありません。 (Kare wa Nihonjin ni chigai arimasen.) - に違いない and に決まっている take a noun bare; だ cannot stand in front of the particle に.
• 日本語が話せないわけがありません。 (Nihongo ga hanasenai wake ga arimasen.) -> 日本語が話せないわけではありません。 (Nihongo ga hanasenai wake de wa arimasen.) - わけがない denies the whole idea (there’s no way I can’t speak it); the intended it’s not that I can’t speak it is わけではない. One particle changes the meaning completely.
• 明日はパーティーに行けません。 (Ashita wa pātī ni ikemasen.) -> 明日はパーティーに行くわけにはいきません。 (Ashita wa pātī ni iku wake ni wa ikimasen.) - use 行けません only if you are genuinely unable; if you are able but a duty or a promise stops you, it is わけにはいきません.

Tips

• はず is arithmetic, に違いない is conviction, でしょう is a guess. Ask yourself what is behind the claim. A timetable or a promise -> はずです. A pile of evidence pointing one way -> に違いありません. Nothing but your instinct -> でしょう. Getting this wrong is not ungrammatical, it just makes you sound either over- or under-confident.
• The whole わけ family is one noun: 訳 “reason”. Read them literally and they stop being confusable: the reason is so (わけだ), no such reason exists (わけがない), it is not that reason (わけではない), that route won’t do (わけにはいかない).
• わけではない is the politeness tool of this lesson. It repairs an inference before it hardens: 高いわけではありませんが、今月はちょっと… (Takai wake de wa arimasen ga, kongetsu wa chotto…, it’s not that it’s expensive, but this month…). Its casual twin わけじゃない is everywhere in speech.
• に決まっている is not a neutral “certainly”. It carries impatience, so it is fine among friends and risky at work. In a meeting, the expected register is the opposite: 〜のではないでしょうか.