Supposition: だろう & でしょう

The volitional form of the auxiliary verb was briefly introduced before when we looked at the volitional.

Form	Dictionary	Volitional
Polite	です	でしょう
Standard	だ	だろう

The volitional for verbs translated as let’s in English; however, for the auxiliary verb the nuance is actually one of supposition.

この問題もんだいを解決かいけつするのは無理むりだ。

Solving this problem is impossible.

この問題もんだいを解決かいけつするのは無理むりだろう。

This problem isn’t likely to be solved.

Both standard and polite forms can be added directly to the end of verbs.

彼女かのじょが頑張がんばれば、できるだろう。

If she tried I reckon she could do it.

沙織さおりが来くると言いったら彼かれも来くるでしょう。

If Saori says she’ll join the study group, I bet he will come, too.

The volitional form of the standard copula can also be added directly to i-adjectives.

それは難むずかしいだろうな。

I bet that’s difficult.

The な particle here is a more masculine ending particle.

Used as a question, でしょうか is also more polite than ですか (again, it has the nuance of “wondering” if something might be okay).

個人的こじんてきなことを聞きいてもいいでしょうか？

I wondering if I may ask something personal?

In fact, we can add a further degree of certainty to this form by removing the う from both the standard and polite forms of the volitional copula.

明日あしたは学校がっこうに行いくでしょ？

You’re going to school tomorrow, right?

やったのはお前まえだろ！

You’re the one who did it!

お前 is a very derogatory way of saying “you”. When this abbreviated form (だろ) is used with the standard form of the volitional it has almost an accusatory tone. In the last example, you know that he’s the one who did it, you just want him to admit it. It has quite a masculine nuance.