でしょう・
Choosing a form for confidence and expectation
Choose by asking what relationship you want to claim between your information and your conclusion. でしょう gives an estimate, はずだ gives an expected result, and わけだ explains why an established result makes sense. The remaining three express strong certainty, but differ sharply in tone: neutral conviction, formal weight, or conversational insistence.
Start with the job the conclusion is doing
These forms do not make a simple ladder from uncertain to certain. First identify the kind of reasoning; then choose a strength and register.
An open estimate
でしょう・だろう
You judge that something is probably true or likely to happen.
This guide covers prediction, not the use that asks the listener to agree.
A result expected from known information
はずだ
A schedule, rule, plan, fact, or other reliable basis makes the result what should follow.
A conclusion that now makes sense
わけだ
Established facts explain an observation or lead naturally to a practical conclusion.
A strongly believed inference
に違いない
Clues lead you to one conclusion that you strongly believe.
A weighty formal certainty
に相違ない
A report, serious commentary, or formal analysis presents a conclusion as beyond doubt.
It normally sounds stiff or document-like in everyday conversation.
An answer treated as obvious
に決まっている
You insist that the answer is settled and competing answers are hardly worth considering.
The force can sound emotional, impatient, critical, or dismissive.
Probability versus an expected result
A ferry may arrive at noon. In the first sentence, the speaker makes a general estimate; in the second, the published schedule provides a firm basis.
風が弱くなったので、船は昼ごろ港に着くでしょう。
The wind has weakened, so the ferry will probably reach the port around noon.
Analyze時刻表では、船は正午に港に着くはずです。
According to the timetable, the ferry should reach the port at noon.
AnalyzeTakeaway: でしょう says what seems likely. はずだ says what the stated information leads you to expect if that information is correct.
Before confirmation and after the facts fall into place
A visitor expects a museum to be open, then learns why it is closed.
案内では、博物館は今日は開いているはずです。
According to the information, the museum should be open today.
Analyze今日は臨時休館日だった。それで、入り口が閉まっているわけだ。
Today was a special closure day. That explains why the entrance is closed.
AnalyzeTakeaway: はずだ projects what should be true from a basis. わけだ looks at established information and says, in effect, “Now the result makes sense.”
Three voices for a strong conclusion
Large footprints suggest that the person who entered was an adult. The conclusion is similar, but the social voice changes.
残された足跡はかなり大きい。侵入したのは大人に違いない。
The footprints left behind are quite large. The person who entered must have been an adult.
Analyze現場の足跡の大きさから判断すれば、侵入者は成人に相違ない。
Judging from the size of the footprints at the scene, there can be no doubt that the intruder was an adult.
Analyzeこんなに大きな足跡だ。入ったのは大人に決まってる。
Look how large these footprints are. Of course it was an adult who went in.
AnalyzeTakeaway: に違いない is the broadly useful strong inference. に相違ない gives the assertion formal rhetorical weight. に決まっている sounds like a speaker personally insisting that the answer is obvious.
The linking form is not shared
Do not assume that these endings attach to nouns and na-adjectives in the same way. For example, a noun can appear as 医者でしょう, 医者のはずだ, or 医者なわけだ. By contrast, the three certainty expressions generally attach directly to the noun or na-adjective stem, as in 医者に違いない. Check the individual member pages for complete formation tables.
More examples across the family
雲が少し出てきたので、夕方は雨になるでしょう。
Some clouds have appeared, so it will probably rain this evening.
Analyze発送の連絡が昨日来たので、荷物は明日届くはずです。
The shipping notice came yesterday, so the package should arrive tomorrow.
Analyzeこの道は朝だけ車が通れない。だから、今はこんなに静かなわけだ。
Cars cannot use this road in the morning. That explains why it is so quiet now.
Analyze机の上に温かいお茶がある。誰かがさっきまでここにいたに違いない。
There is warm tea on the desk. Someone must have been here until just a moment ago.
Analyze以上の調査結果を踏まえれば、この対策は一定の効果を持つに相違ない。
In light of these survey results, there can be no doubt that this measure has a certain effect.
Analyzeあんなに楽しそうに話していたんだ。二人はまた会うに決まってるよ。
They were talking so happily. Of course those two are going to meet again.
Analyze