はず vs べき: Two Kinds of 'Should' (N3 Guide)
The core difference
Both can be voiced as "should" in English, but one describes what you expect to be true and the other what ought to be done:
| Form | Core idea | Example |
|---|---|---|
| 〜はず | Expectation / logical certainty — 'ought to be, based on reason or evidence' | もう着くはずだ = It should arrive by now (I expect it) |
| 〜べき | Obligation / advice — 'should do, the right or proper thing' | 謝るべきだ = You should apologize (it's the right thing) |
はず attaches to a plain-form verb, adjective, or noun (+の) and reports your confident inference. べき attaches to a verb's dictionary form and states a duty or recommendation.
See it in one situation
Same topic — モチ and a promise — but two different kinds of "should":
モチは約束したから、来るはずだ。
もちは やくそくしたから、くる はずだ。
Mochi promised, so he should be coming (I expect him to).
はず — a confident prediction based on the promise. You're not ordering; you're expecting.
約束したんだから、モチは来るべきだ。
やくそくしたんだから、もちは くる べきだ。
He made a promise, so Mochi should come (it's the right thing to do).
べき — an obligation. You're saying it's his duty, not predicting a fact.
The first bets on what will happen; the second judges what ought to happen.
〜はず — expectation and logical certainty
Use はず when you have a good reason to believe something is (or isn't) true. It's not a guess out of thin air — there's evidence behind it.
先生は今日、学校にいるはずです。
せんせいは きょう、がっこうに いる はずです。
The teacher should be at school today (I'm fairly sure).
An expectation grounded in reason — e.g. it's a weekday.
The negative 〜はずがない means "there's no way it's true" — you're confident it's false:
ヤッタンがそんな嘘をつくはずがない。
やったんが そんな うそを つく はずがない。
There's no way Yattan would tell such a lie.
はずがない — strong confidence that something is NOT the case.
Note the contrast: 〜はずだ = "it should be so"; 〜はずがない = "it can't possibly be so." Full details: 〜はず guide.
〜べき — obligation and advice
Use べき to say something is the right or proper thing to do — moral or social correctness, or your opinion about what someone ought to do. It's common in advice.
学生はもっと勉強するべきだ。
がくせいは もっと べんきょうする べきだ。
Students should study more.
べき — an opinion about the right thing to do, not a prediction.
The negative 〜べきではない means "shouldn't (do)" — it's not the proper thing:
人の日記を読むべきではない。
ひとの にっきを よむ べきではない。
You shouldn't read someone else's diary.
べきではない — advice that an action is wrong.
With する, both 「するべき」 and the older 「すべき」 are correct — 勉強すべきだ and 勉強するべきだ mean the same; すべき sounds a touch more formal. Full details: 〜べき guide.
A useful contrast: fact vs duty
- 〜はず = "should be (true)" — you're predicting reality.
- 〜べき = "should (do)" — you're prescribing behavior.
So 弟は家にいるはずだ says you expect your brother is home (a fact you're betting on); 弟は家にいるべきだ says he ought to stay home (a duty you're assigning). Same verb, opposite jobs.
Common mistakes
- Using べき to predict a fact. "The package should arrive tomorrow" is 明日届くはずだ, not 届くべきだ (which would order the package to arrive).
- Using はず for advice. "You should rest" is 休むべきだ, not 休むはずだ (that would mean "you're expected to rest," a prediction).
- Mixing up the negatives. "There's no way that's true" is 〜はずがない; "you shouldn't do it" is 〜べきではない. They are not interchangeable.
- Forcing べき onto nouns/adjectives. べき attaches to a verb's dictionary form. For "it should be true," use はず (which does take nouns + の and adjectives).
Quick recap
- はず = expectation / logical certainty → "should be (true), ought to be."
- べき = obligation / advice → "should (do), the right thing to do."
- はず = I'm sure it's true; べき = it's the right thing to do.
- 〜はずがない = "no way it's true"; 〜べきではない = "shouldn't do."
- With する: するべき = すべき (すべき is slightly more formal).
Your turn
Choose はず or べき for each sentence.
Start the 6-question drill →Frequently asked questions
What's the simplest way to tell はず and べき apart?
Decide which 'should' the English means. A prediction you're confident about ('the train should be here') is はず; a duty or the right thing to do ('you should apologize') is べき.
What does はずがない mean?
It's the negative of はず and means 'there's no way it's true' — strong confidence that something is NOT the case, e.g. そんなはずがない ('that can't be right').
How do I say 'shouldn't' with べき?
Use 〜べきではない, e.g. 読むべきではない ('you shouldn't read it'). It says an action is not the proper thing to do.
Is it するべき or すべき?
Both are correct and mean the same thing. するべき is a little more conversational; すべき is the older, slightly more formal form — 勉強するべきだ = 勉強すべきだ.
The patterns compared here
Full guide for each pattern in this comparison: