Translation guide
In Japanese, expressing an assumption depends on the level of certainty, the basis for the assumption, and whether it is about the past, present, or future. Common patterns include はず for logical expectations, だろう/でしょう for conjecture, and と思う for personal belief. The choice of expression also reflects formality and the speaker's confidence.
Expressing that something is expected to be the case based on reasoning, common sense, or prior information. Often translated as 'should' or 'ought to'.
Indicates a strong assumption based on logical reasoning or factual knowledge. The speaker is confident that something is true or will happen. Used for both present and past assumptions.
彼はもう着いているはずだ。
He should have arrived by now.
If you take this medicine, you should get better.
Used to state a conclusion or assumption that naturally follows from a given situation or explanation. It often translates to 'so that means...' or 'it follows that...'.
彼は日本に10年住んでいたから、日本語が上手なわけだ。
He lived in Japan for 10 years, so it makes sense that his Japanese is good.
Expresses a strong conviction or assumption, often based on intuition or circumstantial evidence. Similar to 'must be' or 'no doubt'.
あの笑顔を見ると、彼は幸せに違いない。
Seeing that smile, he must be happy.
Expressing a personal guess or prediction without strong evidence. Often translated as 'probably', 'I think', or 'maybe'.
Plain form conjecture. Used in casual speech or writing to express a guess. The polite form is でしょう.
明日は雨だろう。
It will probably rain tomorrow.
Polite form of conjecture. Commonly used in formal conversation, news, and polite writing.
明日は雨でしょう。
It will probably rain tomorrow.
Expresses a possibility or uncertain assumption. 'Might' or 'may'. Less confident than だろう/でしょう.
彼は来ないかもしれない。
He might not come.
Literally 'I think that...'. Used to express personal assumption or belief. Can be softened with ～気がする (I have a feeling that...).
彼は成功すると思う。
I think he will succeed.
Expressing what you assume happened or was true in the past, often based on evidence or reasoning.
Assumption about a past event that should have happened according to logic or expectation.
彼は昨日出発したはずだ。
He should have departed yesterday.
Strong assumption about a past event. 'Must have done'.
彼はその秘密を知っていたに違いない。
He must have known the secret.
Possibility about a past event. 'Might have done'.
彼はもう帰ったかもしれない。
He might have already gone home.
Setting up a hypothetical situation or condition for the sake of argument. Often used in formal or academic contexts.
Literally 'assuming that...'. Used to introduce a hypothetical premise.
彼が犯人だと仮定すると、動機は何だろう。
Assuming he is the culprit, what would be the motive?
If we assume that...; supposing that... Used for hypothetical reasoning.
彼が来ないとしたら、どうしよう。
Assuming he doesn't come, what should we do?
The act of taking on a position, role, or responsibility. This is a different sense of 'assumption'.
Assumption of a post or office. Used in formal contexts.
大統領就任の準備を進めている。
Preparations are underway for the assumption of the presidency.
To take on or assume a responsibility or task. More general and can be used in everyday contexts.
はず indicates a logical conclusion based on evidence or reasoning (high certainty). だろう/でしょう is a conjecture or guess (moderate certainty). かもしれない expresses possibility (low certainty). Choose based on how confident you are.
彼は来るはずだ（理由がある）。
He should come (there's a reason).
彼は来るだろう（推測）。
He will probably come (guess).
彼は来るかもしれない（可能性）。
He might come (possibility).
English 'assume' can be translated as 仮定する (to hypothesize) or 思い込む (to take for granted), but these are not always natural. For everyday assumptions, use patterns like はず or と思う instead of trying to directly translate 'assume'.
彼は来ると思っていた。
I assumed he would come.
彼はそのプロジェクトのリーダーを引き受けた。
He assumed the role of leader for the project.