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

By | March 30, 2022

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.

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

  1. Vince

    Great explanation! I was just wondering about the ろ part in だろう. I heard it was the conjugated form of the copula (with the auxiliary verb う/よう at least), with some sound changes. Could you maybe tell us more about where that ろ comes from and how it works?

    1. locksleyu Post author

      According to the dictionary:
      In short, “darou” is made from the “mizenkei” form of the verb “da” (“daro”), plus the auxiliary verb “u” that is used for inference.

  2. Mia

    A somewhat unrelated question to what the article is about – aren’t you supposed to put 「の」after a verb to end it with any variant of 「です」? The example that was featured here was as follows:

    「2時間、運動したから疲れてるでしょう」Wouldn’t this need to be 「2時間、運動したから疲れてるんでしょう」to be grammatically correct? In all the sentences I’ve read, it’s usually written/spoken as either 「疲れてる・疲れています」or 「疲れてるんだ・疲れているのです」.


    1. locksleyu Post author

      Thanks for the question!
      In short, typically するのです(するんです)is correct, while するです is not correct. However, you should not think that the “no” is required to make it grammatically correct, you should think that “no” services a purpose of making something sound like “the fact is”. You can see more details here:


      If you just want to make するです correct you can simply do します。

      Note that しないです is correct, even though of course you can say しません as well.


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