What it means
だろう expresses a guess, expectation, or conclusion based on the speaker’s knowledge or situation. It often means “probably” or “I think,” and it usually follows a plain-form clause. With nouns and な-adjectives, it follows the affirmative form without the final だ. だろう can refer to the present or future, depending on the context.
Usage note
だろう is the plain-form counterpart of でしょう in many sentences, but it is not limited to casual speech. It is also common in writing and formal statements, while in conversation it can sound firm or masculine depending on the speaker and context. Do not normally place a volitional form such as 行こう directly before だろう; that combination is generally unnatural. For a polite version of the same conjecture, use でしょう.
How to use it
これは本でしょう。
This is probably a book.
この部屋は静かでしょう。
This room is probably quiet.
この道は長いでしょう。
This road is probably long.
明日は雨が降るでしょう。
It will probably rain tomorrow.
これは本だろう。
This is probably a book.
この部屋は静かだろう。
This room is probably quiet.
この道は長いだろう。
This road is probably long.
明日は雨が降るだろう。
It will probably rain tomorrow.
Example sentences
明日は雨だろう。
It will probably rain tomorrow.
この店のケーキは人気だろう。
The cakes at this shop are probably popular.
来週の試験は難しいだろう。
Next week's exam will probably be difficult.
電車はもうすぐ来るだろう。
The train will probably come soon.
図書館は午後六時に閉まるだろう。