〜だろう
Probably… — the plain form of でしょうmachine translation
FormationPlain form + だろう (nouns and な-adjectives drop だ)
だろう is the plain form of the でしょう you learned at N5: the speaker guesses at something without being completely certain — "probably, most likely". It often appears with たぶん. Before だろう, verbs and い-adjectives stay in the plain form; nouns and な-adjectives drop だ: 雨だろう, 元気だろう.
With rising intonation at the end — 〜だろう？ — the sentence becomes a confirmation question, like "right?": 難しいだろう？. This usage is like でしょう？ but casual and rather masculine-sounding; in conversation, women more often use でしょ？.
Compared with 〜かもしれない, だろう expresses a clearly higher degree of certainty: 雨が降るだろう is "it will probably rain", while 雨が降るかもしれない is only "it might rain". In writing such as news articles and forecasts, だろう appears very frequently.
Examples
明日は晴れるだろう。
It will probably be sunny tomorrow.
彼はもう家に着いただろう。
He has probably gotten home already.
この問題は難しいだろう？
This problem is really hard, isn't it?