～だろう (conjecture)

• Reading: だろう
• Romaji: darou
• Formation: Verb plain form + だろう / い-Adj + だろう / な-Adj stem + だろう / Noun + だろう

Meaning

An auxiliary indicating the speaker's conjecture or guess that is not based on strong evidence. It often translates as 'probably' or 'I think/suppose.' The polite form is でしょう.

だろう expresses the speaker's personal guess or prediction. It is less certain than はず and less evidence-based than らしい. It follows the plain form of verbs and い-adjectives directly, while な-adjectives and nouns drop だ before だろう. The polite form でしょう is widely used in weather forecasts and polite conversation. When used with a rising intonation or the particle ？, だろう can function as a rhetorical question or seek confirmation ('right?', 'don't you think?'). In casual male speech, だろ is commonly used as a tag question.

Examples

1. 明日はたぶん晴れるだろう。 It will probably be sunny tomorrow.
2. この仕事は来週までに終わるだろう。 This work will probably be finished by next week.
3. 彼女はもう家に着いただろう。 She has probably already arrived home.

Usage Guide

Context: spoken, written, everyday

Tone: speculative

Do Say

• 週末は混むだろうから早めに出よう。
• 彼はきっと合格するだろう。
• 午後から天気がよくなるでしょう。
• そんなことを言ったら怒るだろう。

Don't Say

• 明日は雨だだろう。(Nouns drop だ before だろう: 雨だろう) → 明日は雨だろう。
• 彼女は元気だだろう。(な-adjective stems also drop だ: 元気だろう) → 彼女は元気だろう。
• 昨日買っただろう本はどこ？(だろう cannot modify nouns directly as a relative clause — rephrase with はず or と思う) → 昨日買ったはずの本はどこ？

Origin & History

だろう is the volitional form of the copula だ, which developed from the classical であろう. It shifted from expressing will or intention to marking conjecture during the Edo period.

Cultural Context

Generation: All ages

Social background: Universal