Grammar Point
expressions (phrases, clauses, etc.)
exp 
だろう

seems, I think, I guess, I wonder, I hope

conjectural form of copula だ

Verb or い-Adjective + だろう

The auxiliary だろう is the conjectural form of the copula だ. It indicates the speaker's conjecture or guess, which is often not based on any specific evidence or information. It is less certain than 'probably' and conveys a sense of 'I wonder' or 'I guess.'

Verb (informal form)

When used with verbs, だろう follows the dictionary form or the past tense (た form) to express a guess about a future or past action.

I guess they will come.

I think it rained.

い-Adjective (informal form)

When used with い-adjectives, だろう follows the dictionary form or the past tense form.

I guess it is expensive.

I suppose it was cold.

Note

In casual speech, だろう is often shortened to ろう or replaced by でしょう in polite contexts. While it translates to 'probably,' it often expresses a weaker sense of certainty based on intuition rather than data.

Noun or な-Adjective + だろう

For な-adjectives and nouns, the copula だ is replaced by だろう in the present tense. In the past tense, it follows the informal past form だった.

Noun

In the present tense, the noun is followed directly by だろう. In the past tense, use the informal past だった followed by だろう.

I guess it is a cat.

I suppose it was a mistake.

な-Adjective stem

Similar to nouns, use the stem of the な-adjective for the present tense, and the だった form for the past tense.

I guess it is quiet.

I suppose it was easy.

Note

When using a noun or な-adjective in the present tense, do not include だ before だろう. Saying 'だだろう' is ungrammatical. In casual speech, when in past tense, the だ is often swallowed turning だっただろう into だったろう.

Words: 
だろう
だろ
Radicals of radicals