Translation guide
The English verb 'suppose' has several distinct uses: expressing an assumption or guess, introducing a hypothetical situation, making a suggestion, and indicating obligation or expectation. This guide covers natural Japanese equivalents for each meaning.
To express that you think something is probably true based on evidence or reasoning, but you are not certain.
The most common and versatile way to say 'I think' or 'I suppose.' Used with the plain form of verbs/adjectives plus と思う.
彼は来ると思う。
I suppose he'll come.
それは本当だと思う。
I suppose that's true.
A sentence-ending expression indicating probability or conjecture, similar to 'I suppose' or 'probably.' Often used in casual speech. The polite form is でしょう.
明日は雨だろう。
I suppose it will rain tomorrow.
彼は知っているだろう。
I suppose he knows.
Expresses possibility or uncertainty, like 'might' or 'may.' It can be used for 'I suppose' when the speaker is less confident.
彼は忙しいかもしれない。
I suppose he might be busy.
Indicates a strong expectation based on reasoning, like 'should' or 'ought to.' It implies the speaker is fairly confident.
彼はもう着いているはずだ。
I suppose he should have arrived by now.
To ask someone to imagine a situation or to consider a possibility, often used in discussions or explanations.
A common pattern meaning 'if we suppose that...' or 'assuming that...'. もし is optional but adds emphasis.
もし彼が来なかったとしたら、どうする？
Suppose he doesn't come, what will you do?
仮にそれが本当だとしたら、大変なことだ。
Supposing that's true, it's a serious matter.
A more formal or technical way to say 'suppose' or 'assume.' Often used in academic or logical contexts.
Xが真であると仮定しよう。
Let's suppose X is true.
A casual way to say 'let's suppose' or 'let's assume.' Often used in conversation when proposing a hypothetical scenario.
彼が正しいとしよう。それでどうする？
Suppose he's right. Then what?
To politely suggest an idea or course of action, often using 'I suppose' to soften the statement.
A common way to make a suggestion, equivalent to 'why don't you...?' or 'how about...?' It can convey the nuance of 'I suppose you could...'
彼に聞いてみたらどうですか。
I suppose you could ask him.
Expresses a suggestion with a sense of 'I wish you would...' or 'you should...' It can be used for 'I suppose you should...' in a gentle, regretful tone.
もっと早く来ればいいのに。
I suppose you should have come earlier.
To indicate that someone has a duty or is expected to do something, often used in the phrase 'be supposed to.'
Indicates a rule, arrangement, or expectation, like 'is supposed to.' It implies that something has been decided or is customary.
会議は10時に始まることになっている。
The meeting is supposed to start at 10.
彼は今日来ることになっている。
He's supposed to come today.
Also used for 'supposed to' when expressing a logical expectation or a plan that should happen.
彼女はもう着いているはずだ。
She's supposed to have arrived already.
Expresses moral obligation or strong advice, like 'should' or 'ought to.' It can be used for 'supposed to' when emphasizing duty.
学生は勉強するべきだ。
Students are supposed to study.
While と思う is a direct translation of 'I suppose,' using it too frequently can make your Japanese sound hesitant or uncertain. In many cases, だろう or でしょう is more natural for expressing conjecture.
明日は雨だろう。
I suppose it will rain tomorrow. (more natural than 明日は雨だと思う)
はず indicates an expectation based on logic or arrangement (e.g., 'The train is supposed to arrive at 5'). べき indicates a moral or social obligation (e.g., 'You're supposed to help your parents'). Choose based on whether it's a factual expectation or a duty.
電車は5時に着くはずだ。
The train is supposed to arrive at 5. (factual expectation)
親を手伝うべきだ。
You're supposed to help your parents. (moral obligation)