はずだ

1. 1The speaker judges that something should be or will happen, based on evidence, reasoning, or known facts.Ex. 1
2. 2Something is supposed to be true by general knowledge or information, often followed by a reservation introduced with が or けど.Ex. 2
3. 3The speaker now understands the reason for something: 'no wonder', 'that explains why'.

Used in both conversation and writing, with はずです in polite speech. It states an expectation the speaker believes is logically grounded, so a reason often comes first (for example, a から clause). It is more confident than だろう or かもしれない but less emphatic than に違いない.

• に違いないN3「に違いない」carries the speaker's strong subjective conviction, while 「はずだ」leans toward objective reasoning; as conjectures they are often interchangeable, but the nuance differs. Nouns and な-adjective stems attach directly to 「に違いない」without 「の」or 「な」, and it has no 'was supposed to' usage.
• わけだN3「わけだ」logically connects known facts to a conclusion as an explanation, like saying 'that is to say.' When expressing 'no wonder,' it can be swapped with 「はずだ」; after nouns it can take 「な／の／である」, while 「はずだ」usually takes 「の」.
• はずがないN4「はずがない」is an emphatic negative inference meaning 'there's no way that...', much stronger in tone than 「ないはずだ」('should not be the case'). 「そんなはずがない！」is also commonly said to express disbelief, and the two attach in the same way.
• だろうN5「だろう」also expresses conjecture, but only a subjective, general judgment with less certainty than 「はずだ」; nouns and な-adjectives attach directly to it without 「の」or 「な」. It can also be said with a rising intonation to seek confirmation, but it has no 'was supposed to' usage.
• に相違ないN2「に相違ない」expresses the speaker's strong subjective conviction that something is 'certainly' the case — more assertive than 「はずだ」and mostly used in writing or formal situations; it has no 'was supposed to' usage. Nouns and な-adjectives attach directly or take 「である」.
• かもしれないN4「かもしれない」only signals possibility, with far lower certainty than 「はずだ」, and it does not stress the grounds; it attaches directly to nouns and な-adjective stems without 「の」or 「な」. Swapping the two changes the degree of certainty, and it has no 'was supposed to' usage.

Unlike だろう and かもしれない, which are weaker guesses, はずだ rests on clear grounds, while に違いない (and the formal, written に相違ない) expresses stronger, often intuitive conviction. わけだ presents a conclusion or explanation drawn from facts, and はずがない strongly denies a possibility rather than expecting it.