How to say should to do or must to do in Japanese

「べきだ」 and 「はずだ」 both express an expectation about a situation, but they have different nuances and usage contexts. Below is a detailed comparison:

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1. 「べきだ」 – Expressing Strong Moral or Logical Obligation

Grammar Structure

Verb dictionary form + べきだ
する → するべきだ or すべきだ (both forms are correct, but すべき is more common)

Meaning & Usage

• Expresses a strong sense of obligation, duty, or recommendation.
• It implies that something must be done or should be done, based on moral, logical, or general rules.
• Often used in instructions, personal opinions, or advice.

Examples

✅ 学生は勉強するべきだ。

(Students should study.) → A general rule or expectation for students.

✅ 約束は守るべきだ。

(You should keep your promises.) → A moral obligation.

✅ そんなことを言うべきではない。

(You shouldn’t say such things.) → Expressing strong disapproval.

🚫 Caution: 「べきだ」 expresses a strong sense of obligation, so it may sound too strict or forceful in casual conversations.

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2. 「はずだ」 – Expressing Expectation Based on Reasoning

Grammar Structure

Verb dictionary form / ない-form + はずだ
い-adjective + はずだ
な-adjective / noun + の + はずだ

Meaning & Usage

• Expresses expectation based on logic, experience, or evidence.
• It means “should be” or “I believe it must be like this.”
• It is not about obligation, but about what one believes is true based on some reasoning.

Examples

✅ 彼は今ごろ家にいるはずだ。

(He should be at home by now.) → Based on logical reasoning.

✅ この時計は高かったから、長く使えるはずだ。

(This watch was expensive, so it should last a long time.) → Expectation based on price.

✅ 彼は日本に10年住んでいるから、日本語が話せるはずだ。

(He has been living in Japan for 10 years, so he should be able to speak Japanese.) → Logical assumption.

🚫 Caution: If something contradicts your expectation, you can use: ✅ 「はずがない」 (There is no way…)
✅ 「そんなはずはない」 (That can’t be true!)

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3. Key Differences Between「べきだ」 and 「はずだ」

Aspect	べきだ	はずだ
Meaning	“Should do / Must do”	“Should be / Must be”
Focus	Obligation or recommendation	Expectation or logical assumption
Context	Giving advice, expressing duty or responsibility	Making predictions based on facts or logic
Example 1	学生は勉強するべきだ。 (Students should study.)	彼は今勉強しているはずだ。 (He should be studying now.)
Example 2	人を傷つけるべきではない。 (You shouldn’t hurt people.)	彼はそんなことを言うはずがない。 (He would never say such a thing.)
Tone	Stronger, more directive	Softer, more about belief or expectation

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4. When They Cannot Be Interchanged

❌「べきだ」 Cannot Replace「はずだ」

✅ 彼は今家にいるはずだ。 (He should be at home now.)
❌ 彼は今家にいるべきだ。 (Sounds unnatural unless implying obligation.)

❌「はずだ」 Cannot Replace「べきだ」

✅ 約束は守るべきだ。 (You should keep your promises.)
❌ 約束は守るはずだ。 (This means “I expect you to keep your promises,” which has a different meaning.)

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5. Summary

• 「べきだ」 = Obligation / Duty / Moral expectation → “should do” / “must do”
• 「はずだ」 = Logical expectation / Prediction → “should be” / “must be true”

💡 If you want to give advice or set rules, use べきだ.
💡 If you want to make logical assumptions, use はずだ.