Japanese Grammar: The mysterious connection between the volitional form and でしょう / だろう (deshou / darou)

In this post, I wanted to discuss an interesting connection between the volitional form in Japanese and the words “deshou” / “darou”. To begin with I will go over the meaning of each, and then see how they relate.

The volitional form represents the speaker’s volition, or will, and for the verb “suru” looks like “shiyou” (regular form) / “shimashou” (polite form). Sometimes I call this the “let’s form” since it is easy to remember. Conjugation for other verb types generally ends with a “~ou” or “~you” sound (ex: aruku -> arukou, taberu -> tabeyou, nomu -> nomou, etc.) Here is an example of usage:

• ランチはサンドイッチを食べましょう (ranchi wa sandoicchi wo tabemashou)
• For lunch let’s have sandwiches.

For more details on this form see this article.

The words “deshou” (polite) and “darou” (regular) can be used with a few different shades of meaning depending on the context and the speaker’s voice, but generally they represent an inference or guess (推量 [suiryou]) of the speaker/writer. Let’s look at an example:

• ２時間、運動したから疲れてるでしょう (ni jikan, undou shita kara tsukarete ru deshou)
• (You) exercised for two hours, so you must be (are probably) tired.

See this article for more details and example sentences on deshou/darou.

For many years I thought these things were totally unrelated, but then recently I had a mini “eureka” moment where I realized their connection. To see how, we need to dig a little deeper into some grammar details.

It turns out that the volitional form is actually created using a form (未然形 [mizenkei], or imperfective form) of the verb plus the “u”（う) or “you” (よう) auxiliary verb (助動詞 [jodoushi]). So for 食べよう (tabeyou), the “tabe” part would be the imperfective form, and the “you” the auxiliary verb. In this case, the “u”/”you” auxiliary verb represents volition/will (意志 [ishi]), but it can also be used for an invitation (勧誘 [kan’yuu]).

Now if we come back to “deshou” / “darou”, it so happens that these words are actually also in the same form: [verb in imperfective form] + [u/you auxiliary verb]. The confusing part is that the “u/you” auxiliary verb has another meaning which is inference (推量), which is consistent with our understanding of the “deshou” / “darou” words.

Getting down and dirty with grammar is fun at all, but how does understanding this actually help? Well, it turns out there is a certain classical usage that inspired me to make this connection in the first place. Let’s look at an example of this usage:

• そんなこと、本当にありましょうか (sonnna koto, hontou ni arimashou ka)

If we try and interpret this sentence with basic Japanese, we end up with “arimashou” meaning the polite volitional form of “aru” (to exist for inanimate things). But what does it mean to express the “will” of “existing”? A literal translation would be something like “let’s exist” (or “let’s be”) which doesn’t make sense.

However, with our newfound knowledge of the auxiliary verb “u”/”you” and how it can also mean “inference”, we can interpret this in a different (and more correct) way, what can be explained in more modern Japanese this way:

• そんなこと、本当にあるでしょうか (sonnna koto, hontou ni aru deshou ka)
• Is such a thing really possible?

Do you have any questions about Japanese grammar? Feel free to let me know in the comments.