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It's said that 一段 are called 一段 because they keep their stem, but is that really true?

Like, in 終止形, the verb has る, which is ウ段. And then, in 命令形, it has ろ, which is オ段.

How do you explain this?

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    る isn't part of the stem, though...?
    – Leebo
    Feb 26, 2023 at 13:55
  • i was thinking about that too, but, what it is then? as well as ろ
    – sieman
    Feb 26, 2023 at 16:55

2 Answers 2

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Its said that 一段 are called 一段 because they keep their stem.

You just answered your own question. 一段 verbs keep their stems intact, while the suffix change to reflect different conjugations. For example:

  • 食べる dictionary form
  • 食べます respectful form
  • 食べない negative form
  • 食べません negative-respectful form
  • 食べた past tense
  • 食べれば if-form
  • 食べろ imperative form
  • 食べさせる causative form
  • 食べられない negative-potential form
  • The list goes on and on...

You can see how in each of these forms the stem 食べ never changes. Different endings are added, which happen to include る for dictionary form and ろ for imperative. For 五段, let's see a comparison:

  • dictionary form
  • ます respectful form
  • ない negative form
  • ません negative-respectful form
  • いた past tense
  • ば if-form
  • imperative form
  • せる causative form
  • ない negative-potential form
  • The list goes on and on...

While the stem itself is 歩き, the き could change to か き く け or こ depending on which 段 the conjugation needs to be in. There are five in total(かきくけこ) hence the name 五段 verbs.

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    It might be worth pointing out that 歩こう is the one form that in fact makes these verbs 五段 in the modern language and not 四段.
    – jogloran
    Feb 27, 2023 at 4:57
  • So, い (for the past tense) is not included in the calculation?
    – qrsngky
    Feb 27, 2023 at 10:30
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    @qrsngky With ~く and ~ぐ turning into い, that's a mutation of the 連用形, as is ~った for る and つ. Strictly speaking the volitional's ~おう is from a mutation of the 未然形 + む・う, but unlike the others it gets counted
    – Angelos
    Feb 27, 2023 at 15:29
  • @jogloran Yes should've added 歩こう there to demonstrate all five. Thanks for the addition
    – dvx2718
    Feb 27, 2023 at 16:03
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dvx2718's answer should be canonical. I just add to clarify the point (I think) you are confused about.

Take 切る(五段) and 切れる(下一段) for examples.

In my understanding, 切り1/切れ1 are both masu-stem in learners' grammar. Note 切1 of 切る is a conjugating part of 切る and 切れ1 is the NON-conjugating part of 切れる. This may well make you think that れ etc. is part of stem. But (at least in some contexts), this is not the case, as mentioned in this Wikipedia article (which should apply to 下一段 as well with obvious changes).

言語学から言えば、上一段活用の動詞は語幹が母音で終わる母音語幹動詞である。そのため語幹は「る」の前のイ段母音までとされ、それ以後が語尾とされる

So the bolded る・る・れ・ろ in the following table is 語尾 (common to all Group 2 verbs), and not stem when you talk about 段. In this context 切れ is the only part called stem, hence the name ichidan.

五段 下一段
未然形 ら・ろ
連用形 1 1
終止形
連体形
仮定形
命令形

Kind of related. I think there are vagueness/inconsistencies in terminology.

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    Where does 切り fit here?
    – qrsngky
    Feb 27, 2023 at 10:32
  • @qrsngky It's a typo. Thank you.
    – sundowner
    Feb 27, 2023 at 10:41

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