JLPT N3・N4 Grammar – Difference between “〜べきだ” “〜はずだ” and “〜にちがいない”
Table of Contents
1. Difference between “〜べきだ” “〜はずだ” and “〜にちがいない”
2. 〜べきだ
3. 〜はずだ
4. 〜にちがいない
5. Summary
6. Similer Articles
7. Comment
Q: What is the difference between “〜べきだ,” “〜はずだ,” and “〜にちがいない”?
A: These three expressions may seem similar, but they differ in nuance. Let’s take a closer look at each.
〜べきだ (JLPT N3)
[Meaning]
Indicates an action that is naturally expected of a person, or considered morally or socially “the right thing to do.”
Often used for strong suggestions or advice.
[Rule]
[V] Verb dictionary form + べきだ
Note: For “する,” both “するべき” and “すべき” are acceptable forms.
[Examples]
約束は守るべきです。
You should keep your promises.
子供には正しいことを教えるべきです。
You should teach children what is right.
他人の話は最後まで聞くべきだよ。
You should listen to others until they finish speaking.
社会の一員として、ルールは守るべきだと思います。
As a member of society, I think you should follow the rules.
“べきだ” expresses a moral or social obligation—what someone should or ought to do as a person. It’s often used to give advice or make strong suggestions.
[Examples]
貴重品を拾ったら、すぐに警察に届けるべきです。(Obligation)
If you find a valuable item, you should report it to the police immediately.
会社の話を他人にすべきではない。(Obligation)
You shouldn’t talk about company matters with outsiders.
一度やると言ったのなら、最後までやるべきだよ。(Strong suggestion)
If you said you would do it, you should see it through to the end.
その問題は会社全体で考えるべきことだね。(Strong suggestion)
That problem is something the whole company should consider.
〜はずだ (JLPT N4)
[Meaning]
① Indicates a logical or objective inference: “It must be so” based on evidence or reason.
② Expresses a natural or expected result given the circumstances or facts.
[Rule]
[V] Verb plain form + はずだ
[A] い adjective plain form + はずだ
[Na] な adjective + はずだ / である + はずだ
[N] Noun + の + はずだ / である + はずだ
[Examples]
電気がついているから、誰かいるはずだよ。（①）
The lights are on, so someone must be here.
ここから駅までは1時間ぐらいかかるから、到着は16時ごろになるはずです。（①）
It takes about an hour from here to the station, so we should arrive around 4 p.m.
寒いはずだよ。雪が降っている！（②）
No wonder it feels cold. It’s snowing!
鈴木さんは一人で仕事をしているんだから、大変なはずです。（②）
Since Mr. Suzuki is working alone, it must be tough for him.
“はずだ” has two primary uses:
① Strong inference based on objective grounds
This is used when making a strong assumption based on facts, data, or other objective information.
[Examples]
（警察）西側の防犯カメラに犯人が映っている。犯人は西に向かって逃げたはずだ。
(Police) The suspect was captured on the security camera on the west side. The suspect must have fled west.
⇒ A logical inference based on objective evidence that the suspect went west.
Googleマップによると、私たちはもう店の近くにいるはずだよ。
According to Google Maps, we should already be near the store.
⇒ The speaker is confident due to the objective reference (Google Maps).
② Natural result or expectation based on facts or circumstances
This is used when the speaker expresses a result that naturally follows from a certain situation, or when conveying their expectation.
[Examples]
山の上は気温が低いから、今ごろは雪が積もっているはずだよ。
The temperature is low at the top of the mountain, so there must be snow piled up by now.
⇒ This is a logical inference based on general knowledge that higher places are colder.
Ａ：このケーキは三ツ星レストランのケーキなんです。
Ｂ：ああ、じゃあおいしいはずだ。
A: This cake is from a three-star restaurant.
B: Oh, then it must be delicious.
⇒ The speaker expresses their expectation that a cake from a famous restaurant would naturally be delicious.
〜にちがいない (JLPT N3)
[Meaning]
This is a subjective expression used when the speaker strongly believes or is certain that something is surely true.
[Rule]
[V] Verb plain form + にちがいない
[A] い adjective plain form + にちがいない
[Na] な adjective + にちがいない / な adjective + である + にちがいない
[N] Noun + にちがいない / Noun + である + にちがいない
[Examples]
母は昨日のことで怒っているにちがいありません。
My mother must be angry about what happened yesterday.
こんなにチョコレートや砂糖を使ったケーキ、とても甘いにちがいないです。
A cake made with this much chocolate and sugar must be very sweet.
あの人はお金持ちにちがいないね。
That person must be rich.
彼が犯人にちがいないよ。
He must be the culprit.
“にちがいない” is used when the speaker makes a definitive guess based on intuition or situational judgment, meaning “there’s no doubt that 〜.” It is more subjective and expresses stronger certainty than “はずだ.”
[Examples]
部屋に足あとが残っている。誰かが入ったにちがいない。(A conviction based on objective circumstances)
There are footprints left in the room. Someone must have entered.
彼のカバンから盗まれた品が出てきた。彼がやったにちがいない。(A conclusion based on evidence)
The stolen item came out of his bag. He must have done it.
ずっと連絡がないなんて…何かあったにちがいない。(A guess based on worry or anxiety)
No contact at all for such a long time… Something must have happened.
あんなに嬉しそうな顔をしてたんだから、試験に合格したにちがいないよ。(A guess based on his expression or behavior)
He looked so happy—he must have passed the exam.
Summary
| Meaning | Nuance | Degree of Certainty | |
|---|---|---|---|
| 〜べきだ | Something one ought to do as a person; a moral or social obligation | Expresses moral duty or a natural course of action | – (Not used for inference) |
| 〜はずだ | ① Logical inference based on objective facts or circumstances ② A result or outcome that is expected as natural | Objective, logical reasoning | Medium |
| 〜にちがいない | It must be 〜 / There’s no doubt 〜 | Strong conviction based on subjectivity, intuition, or impression | High |
Similer Articles
- JLPT N3 Vocabulary – “さける” or “よける”? The Meanings and Usage Differences of “避ける”
- JLPT N3 Grammar – Difference between “〜向き” and “〜向け”
- JLPT N4 Vocabulary – “あく” or “すく”? The Meanings and Usage Differences of “空く”
- JLPT N3・N4 Grammar – Difference between “〜べきだ” “〜はずだ” and “〜にちがいない”
- JLPT N4 Grammar – The Complete Guide to Passive, Causative, and Causative-Passive Form ③
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