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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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grammar ichidan-verbs
Why is 一段 called the 一段?
> 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:

  * 歩 **く** dictionary form
  * 歩 **き** ます respectful form
  * 歩 **か** ない negative form
  * 歩 **き** ません negative-respectful form
  * 歩 **いた** past tense
  * 歩 **け** ば if-form
  * 歩 **け** imperative form
  * 歩 **か** せる causative form
  * 歩 **け** 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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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
**り** 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 **る** etc. is
part of stem. But (at least in some contexts), this is not the case, as
mentioned in [this Wikipedia
article](https://ja.wikipedia.org/wiki/%E4%B8%8A%E4%B8%80%E6%AE%B5%E6%B4%BB%E7%94%A8)
(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 | れ  
連用形 | り1 | れ1  
終止形 | る | れ **る**  
連体形 | る | れ **る**  
仮定形 | れ | れ **れ**  
命令形 | れ | れ **ろ**  
  
* * *

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

  * [Conflicting Definitions of Verb “Roots,” “Bases,” and “Stems”](https://japanese.stackexchange.com/q/96150/45489)
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answer