かもしれない vs でしょう vs はずだ: How to Express Probability and Certainty in Japanese

You want to say “maybe it will rain” — but Japanese gives you at least three different ways to do it. かもしれない, でしょう, and はずだ all express some kind of uncertainty or prediction, yet they carry very different nuances. Mix them up and you will sound unnatural, or worse, overconfident when you should be hedging.

The good news: once you understand the single key idea behind each expression — how certain you are, and whether you have a reason for that certainty — the choice becomes straightforward.

This guide covers all three expressions with real examples, a side-by-side comparison table, a decision flowchart, and a quick quiz to lock in your understanding.

At a Glance

Key insight: The three expressions sit on a certainty spectrum. はずだ is not just “more confident” than かもしれない — it specifically implies you have a reason (evidence, logic, a prior agreement) for your expectation.

1. What Are These Expressions? Formation Notes

Before diving into nuance, let’s look at how each expression is formed so you can build your own sentences.

かもしれない

Attach かもしれない directly to the plain form (dictionary form) of a verb, い-adjective, or noun/な-adjective.

Formation note for N5 readers: The “plain form” of a verb is the dictionary form you look up — 食べる, 行く, etc. For past tense, use the plain past form: 食べたかもしれない (“might have eaten”).

でしょう

でしょう follows the same pattern as かもしれない. It is the polite equivalent of だろう (casual/informal).

Note: だ before でしょう is usually dropped for nouns and な-adjectives in natural speech (先生でしょう, not 先生だでしょう).

はずだ

はずだ follows a noun-modifying (attributive) form — meaning you attach はずだ as if はず were a noun being modified.

Formation note: For past expectations, use the plain past form before はずだ: 届いたはずだ (“it should have arrived”).

2. かもしれない — “Maybe / It Might Be” (Low Certainty)

What It Means

かもしれない expresses a low-to-medium possibility — you think something might be true or might happen, but you are not committed to it. There is no requirement to have evidence. It is simply a personal, open-ended guess.

In English, it maps most naturally to:
– “Maybe…”
– “It might be…”
– “Perhaps…”
– “It’s possible that…”

Examples

1. 明日、雨が降るかもしれない。
   Ashita, ame ga furu kamoshirenai.
   “It might rain tomorrow.”
   (You glanced at the sky. No weather app. Just a feeling.)

2. 彼女は今、忙しいかもしれない。
   Kanojo wa ima, isogashii kamoshirenai.
   “She might be busy right now.”
   (Her phone is off. You are not sure why.)

3. このカフェ、閉まっているかもしれない。
   Kono cafe, shimatte iru kamoshirenai.
   “This cafe might be closed.”
   (You are walking up to it and notice it looks dark inside.)

4. 彼はもう知っているかもしれない。
   Kare wa mou shitte iru kamoshirenai.
   “He might already know.”

5. 道を間違えたかもしれない。
   Michi wo machigaeta kamoshirenai.
   “I might have taken the wrong road.”
   (Past tense — using plain past form 間違えた before かもしれない.)

6. それは本当じゃないかもしれない。
   Sore wa hontou ja nai kamoshirenai.
   “That might not be true.”

Nuance Note

かもしれない leaves the door open — you are not claiming the thing is likely. If you said 雨が降るかもしれない to a native speaker, they would understand you have maybe a 30–50% personal estimate. It carries no implication that you checked anything.

3. でしょう — “Probably / I Think” (Medium Certainty)

What It Means

でしょう (and its casual equivalent だろう) expresses a moderate-to-high probability based on the speaker’s intuition, general knowledge, or impression of a situation. You are not certain, but you lean toward the prediction being correct.

In English:
– “Probably…”
– “I think…”
– “It’s likely that…”
– “I suppose…”

Examples

1. 明日も暑いでしょう。
   Ashita mo atsui deshou.
   “It will probably be hot tomorrow too.”
   (Said by a weather forecaster — or just someone who has been in Japan in August.)

2. 田中さんはもう家に帰ったでしょう。
   Tanaka-san wa mou ie ni kaetta deshou.
   “Mr. Tanaka has probably gone home already.”
   (It is 9 pm. He usually leaves at 6.)

3. この問題は難しいでしょう。
   Kono mondai wa muzukashii deshou.
   “This problem is probably difficult.”

4. 彼女も来るでしょう。
   Kanojo mo kuru deshou.
   “She will probably come too.”

5. このお店は有名でしょう。
   Kono omise wa yuumei deshou.
   “This shop is probably well-known.”

6. 試験には合格できるでしょう。
   Shiken ni wa goukaku dekiru deshou.
   “You can probably pass the exam.”

でしょう vs だろう: Formality

Watch out: でしょう? (rising intonation) changes the meaning entirely — it becomes a tag question (“…right?” / “isn’t it?”). This is different from the assertive でしょう (falling intonation) used for predictions.

暑いでしょう？ → “It’s hot, right?”
明日は暑いでしょう。→ “It will probably be hot tomorrow.”

4. はずだ — “It Should Be / I Expect” (High Certainty + Evidence)

What It Means

はずだ expresses a strong expectation based on logical grounds, prior knowledge, or a previous agreement. You believe something is the case because you have a reason to believe it — not just a feeling.

In English:
– “It should be…” (logical expectation)
– “It must be…” (confident inference)
– “I expect that…” (based on known facts)
– “It ought to be…”

The critical difference from かもしれない and でしょう: はずだ requires a basis. If someone asks “why do you think so?” you should have an answer.

Examples

1. 電車は10時に着くはずだ。
   Densha wa juu-ji ni tsuku hazu da.
   “The train should arrive at 10.”
   (You checked the timetable. This is a factual expectation.)

2. 彼女はもう知っているはずです。
   Kanojo wa mou shitte iru hazu desu.
   “She should already know.”
   (You told her yourself last week.)

3. この薬を飲めば、熱は下がるはずです。
   Kono kusuri wo nomeba, netsu wa sagaru hazu desu.
   “If you take this medicine, the fever should go down.”
   (Medical reasoning — the doctor believes based on how the medication works.)

4. 鍵はここにあるはずなのに、見つからない。
   Kagi wa koko ni aru hazu na no ni, mitsukaranai.
   “The key should be here, but I can’t find it.”
   (You put it here yesterday. Your expectation meets reality, and reality wins.)

5. 彼はもう着いたはずだ。
   Kare wa mou tsuita hazu da.
   “He should have arrived by now.”
   (His flight landed two hours ago.)

6. そんなことはあるはずがない。
   Sonna koto wa aru hazu ga nai.
   “That can’t be right.” / “There is no way that’s true.”
   (This uses はずがない — negating the expectation entirely. It expresses strong disbelief based on what you know to be true.)

はずがない vs はずではない

5. Side-by-Side Comparison and Decision Flowchart

Comparison Table

Decision Flowchart

[You want to express a prediction or expectation]
            |
            v
Do you have a specific reason / evidence / prior knowledge?
    |                                  |
   YES                                NO
    |                                  |
    v                                  v
Use はずだ                   How confident are you?
"It should be / must be"        |              |
(based on logic or fact)      LOW           MEDIUM-HIGH
                               |              |
                               v              v
                        かもしれない        でしょう
                        "Maybe…"         "Probably…"
                        "It might be…"   "I think…"

Example walk-through:
– “I think Yuki is sick” (just a feeling from her tired voice) → でしょう
– “Yuki might be sick” (pure guess, no reason) → かもしれない
– “Yuki must be sick — she called in sick this morning” → はずだ

6. Common Mistakes by English Speakers

Mistake 1: Using はずだ for all “must be” situations

English “must be” covers two very different meanings:
– Logical deduction: “He must be home — the lights are on.” → はずだ (correct)
– Obligation: “You must be joking.” → This is NOT はずだ territory

But more importantly, English “must be” can also be based on feeling rather than hard evidence. In Japanese, if you don’t have a clear reason, don’t use はずだ. Use でしょう instead.

“He must be smart.” (just an impression) → 頭がいいでしょう, NOT 頭がいいはずだ
“He must be smart — he got into Tokyo University.” → 頭がいいはずだ (now you have evidence)

Mistake 2: Overusing かもしれない to sound polite/humble

Some learners use かもしれない constantly to avoid sounding too assertive. While it is polite to hedge, using かもしれない when でしょう or はずだ is appropriate can make you sound unsure or untrustworthy.

Saying “明日、会議があるかもしれない” when you know there is a meeting makes it sound like you have forgotten, or you did not check your calendar. はずだ or just stating the fact is more natural.

Mistake 3: Confusing でしょう with a question

As noted earlier, でしょう with a rising intonation (or written as でしょう？) is a confirmation-seeking tag question — not a prediction. This catches English speakers off guard because English “probably” is never a question.

明日、来るでしょう。→ “You will probably come tomorrow.” (prediction)
明日、来るでしょう？→ “You’re coming tomorrow, right?” (asking for confirmation)

Mistake 4: Forgetting the negative forms

Each expression has a different negative structure — and the meaning shifts significantly.

Quick Quiz

Fill in the blank with かもしれない, でしょう, or はずだ. Check your answers below.

Question 1:
明日のパーティー、田中さんも来る _____。
彼女を招待したから。

Ashita no paatii, Tanaka-san mo kuru _____.
Kanojo wo shoutai shita kara.
(“She’s been invited, so she ___ come to tomorrow’s party.”)

Question 2:
空が曇っているね。雨が降る _____。

Sora ga kumotte iru ne. Ame ga furu _____.
(“The sky is cloudy. It ___ rain.”)

Question 3:
この映画、面白い _____。みんながすごくほめていたから。

Kono eiga, omoshiroi _____. Minna ga sugoku homete ita kara.
(“This movie ___ be good — everyone was praising it highly.”)

Answers

Answer 1: はずだ
You invited her. That is a concrete reason to expect she will come. はずだ expresses an expectation grounded in a known fact (the invitation). でしょう would work too but feels weaker — はずだ is more natural here because “から” (because) signals evidence.

Answer 2: かもしれない
You are looking at the cloudy sky — it is a personal observation with no certain outcome. There is no guarantee of rain. かもしれない fits “might rain” perfectly. Note: if you had checked a weather forecast, でしょう would be more natural (you would have evidence for a moderate prediction). Without checking, かもしれない is the safer, more honest choice.

Answer 3: はずだ
“Because everyone praised it highly” is the reason. You have evidence for your expectation → はずだ. If you had no reason and just guessed it was good, でしょう or かもしれない would be better.

Summary Table

Keep Learning

Now that you can express degrees of certainty in Japanese, you might find these related guides useful:

• If you are still sorting out Japanese conditional forms, see our guide on 〜ても vs 〜たら — two “if” expressions that English speakers frequently mix up.
• For another pair of expressions about change and intention, check out 〜ようになる vs 〜ようにする.
• Want to master obligation and permission? Read 〜なければならない vs 〜なくてもいい.

Published on JPyokoso — practical Japanese for real learners.