Ichidan Verbs

In Japanese, ichidan verbs are verbs that end in ~eru and ~iru, which undergo ichidan katsuyou 一段活用, "one-column conjugation." This means that, when conjugated, their stem ends at the vowel, ~e or ~i, and it's always that same one vowel, no matter what suffixes are attached to it.

They're also called "group 2 verbs," among the three groups of verbs that exist in Japanese.

上一段・下一段

The term "ichidan verb" actually refers to two different types of verbs:

kami-ichidan
上一段
Upper ichidan, which end in ~iru.
shimo-ichidan
下一段
Lower ichidan, which end in ~eru.

For example, niru 似る, "to resemble," is a kami-ichidan verb, while neru 寝る, "to sleep," is a shimo-ichidan verb.

In modern, standard Japanese, there's absolutely no difference in how these two types are conjugated.

In some dialects, there are differences. Historically, there are differences, too. But, in general, there's no difference. They're the same thing.

Conjugation

The ichidan verbs always end in ~eru or ~iru. In other words, they always end in ~ru ～る. Their conjugation is done by removing the ~ru ending and replacing it by something else.

The six basic forms of a kami-ichidan verb would be:

~i ～i
mizenkei 未然形.
~i ～i
ren'youkei 連用形.
~iru ～iる
shuushikei 終止形.
~iru ～iる
rentaikei 連体形.
~ire ～iれ
kateikei 仮定形, or izenkei 已然形.
~iro ～iろ
~iyo ～iよ
meireikei 命令形

A shimo-ichidan verb is the same thing, except the stem is ~e, not ~i:

~e ～e
~e ～e
~eru ～eる
~eru ～eる
~ere ~eれ
~ero ~eろ
~eyo ～eよ

Note that the six basic forms include the suffixes ~ru ～る, ~re ～れ, ~ro ～ろ, and ~yo ～よ. However, the stem which all six forms branch from is the vowel ~i or ~e. Therefore, they're vowel-stem verbs.

Since it's always the same one vowel, they're called ichidan 一段, "one column." This contrasts with godan 五段 verbs, which have a consonant stem, and the vowel changes across all five columns: a-i-u-e-o.

Like other inflectable words in Japanese, ichidan verbs are conjugated by agglutinating these six basic forms with jodoushi 助動詞 suffixes, and other auxiliaries. Observe:

taberu
食べる
To eat. (non-past form, predicative form, shuushikei, attributive form, rentaikei.)
tabeta
食べた
Ate. (past form.)
- tabetara
  食べたら
  If ate. (tara-form.)
- tabetari
  食べたり
  Ate and. (tari-form.)
tabenai
食べない
To not eat. (negative form.)
- tabenakatta
  食べなかった
  Did not eat. (past negative form.)
tabe
食べ
Eating. (noun form, ren'youkei.)
To eat and.
tabemasu
食べます
To eat. (polite form.)
- tabemashita
  食べました
  Ate. (past polite form.)
- tabemasen
  食べません
  To not eat. (negative polite form.)
  - tabemasen deshita
    食べませんでした
    Didn't eat. (past negative polite form.)
tabete
食べて
To eat and. (te-form.)
tabetai
食べたい
To want to eat. (desiderative tai-form.)
- tabetakatta
  食べたかった
  Wanted to eat. (past desiderative form.)
- tabetakunai
  食べたくない
  To not want to eat. (negative desiderative form.)
  - tabetakunakatta
    食べたくなかった
    Did not want to eat. (negative past desiderative form.)
taberareru
食べられる
To be eaten. (passive form.)
To be able to eat. (potential form.)
- tabereru
  食べれる
  To be able to eat. (potential form, ra-nuki-kotoba ら抜き言葉.)
- taberareta
  食べられた
  Was eaten. (past passive form.)
  Could eat. (past potential form.)
- taberarenai
  食べられない
  Is not eaten. (negative passive form.)
  Isn't able to eat. (negative potential form.)
tabesaseru
食べさせる
To cause to eat. (causative form.)
To make [someone] eat [something].
To let [someone] eat [something].
- tabesasenai
  食べさせない
  To not let [someone] eat [something]. (negative causative form.)
- tabesaserareru
  食べさせられる
  To be caused to eat. (passive causative form.)
  To be made eat [something] by [someone].
  To be let eat [something] by [someone].
tabeyou
食べよう
Let's eat. (volitional form.)
tabero
食べろ
Eat. (imperative form, meireikei.)
tabenasai
食べなさい
Eat. (imperative nasai-form.)
tabereba
食べれば
If eat. (conditional ba-form.)
tabenu
食べぬ
To not eat. (negative nu-form.)
- tabeneba
  食べねば
  If not eat. (conditional ba-form on nu-form.)
tabezu
食べず
Without eating. (zu-form.)

In order to conjugate another ichidan verb, like neru 蹴る, just remove the ~ru of the verb, remove tabe~ from the above, and join the two things. For example, the ba-form of taberu is tabe-reba, so the ba-form of neru must be ne-reba.

The same applies to kami-ichidan verbs: ni-reba, ni-ta, ni-nai, ni-masu, ni-you, and so on.

Besides the forms above, there are other forms created from auxiliary verbs and auxiliary adjectives.

Those that attach to the ren'youkei create compound words:

tabe-hajimeru
食べ始める
To start eating.
- A compound verb, fukugou-doushi 複合動詞, with the auxiliary verb hajimeru, "to start."
tabe-yasui
食べやすい
Easy to eat.
- A compound adjective, fukugou-keiyoushi 複合形容詞, with the auxiliary adjective yasui, "easy."

The hojo-doushi 補助動詞 and hojo-keiyoushi 補助形容詞 are those that attach to the te-form.

tabete-iru
食べている
To be eating.
- te-iru form.
tabete-oku
食べておく
To eat [in preparation for something].
- te-oku form.
tabete-ageru
食べてあげる
To eat [for someone].
- ageru あげる auxiliary verb of giving.
tabete-kudasai
食べてください
Please eat [for me]..
- kudasai ください auxiliary verb of requesting.
tabete-hoshii
食べてほしい
[I] want [you] to eat [something].
- hoshii ほしい auxiliary adjective.

With this we've covered pretty much all forms of an ichidan verb.

Dialects

In some regions of Japan, shimo-ichidan and kami-ichidan verbs have different conjugations.

For example, in Kyoto 京都, a negative form of ichidan verbs ending in ~eru gets an ~ehen ending, while a negative form of ichidan verbs ending in ~iru gets an ~ihin ending.

taberu
食べる
To eat.
tabehen
食べへん
To not eat.

okiru
起きる
To happen.
okihin
起きひん
To not happen.

History

In classical Japanese, there's only ONE shimo-ichidan verb: keru.

This leads some resources to say that keru is the only shimo-ichidan verb at all, which is kind of confusing, since taberu 食べる, "to eat," oshieru 教える, "to teach," ukeru 受ける, "to receive," semeru 攻める, "to attack," and so on are all ichidan verbs that end in ~eru.

Even more confusing yet, if you check the dictionary, it will say that keru 蹴る is a godan verb, not an ichidan verb. What's up with that? Why would someone lie so baldly and boldly like that?

Well, it turns out that classical Japanese isn't modern Japanese.

The only shimo-ichidan verb in classical Japanese is a godan verb in modern Japanese. So what are the shimo-ichidan verbs in modern Japanese? They're shimo-nidan 下二段 verbs in classical Japanese.

Furthermore, kami-ichidan and kami-nidan 上二段 verbs in classical Japanese have all turned into kami-ichidan in modern Japanese.

As the name implies, nidan verbs conjugate to "two columns." The conversion from nidan to ichidan verbs was done simply by conjugating the nidan verb to its ren'youkei form, which ends with ~i or ~e, and adding a ~ru to it. Observe:

iku
生く
(a kami-nidan verb.)
iki
生き
(its ren'youkei form.)
ikiru
生きる
To live.
(the modern kami-ichidan version.)

nu
寝
(a shimo-nidan verb.)
ne
寝
(its ren'youkei form.)
neru
寝る
To sleep.
(the modern shimo-ichidan version.)

Auxiliaries

Sometimes, things that aren't exactly verbs are conjugated like ichidan verbs, too.

This happens with the passive and causative jodoushi, ~rareru and ~sareru, and with the potential one, too, specially the potential form of godan verbs. Observe:

tabe-ru
食べる
To eat.
tabe-rare-ru
食べられる
To be eaten. (passive.)
To be able to eat. (potential.)

tabe-nai
食べない
To not eat.
tabe-rare-nai
To not be eaten.
To not be able to eat.

tabe-ta
食べた
Ate.
tabe-rare-ta
食べられた
Was eaten.
Was able to eat.

In godan verbs, the potential form is supposed to end in ~areru, but in practice it actually ends in ~eru.

koros-u
殺す
To kill.
koros-are-ru
殺させる
To be killed. (passive)
To be able to kill. (potential.)
koros-e-ru
殺せる
To be able to kill. (potential.)

This one is conjugated like an ichidan verb, too.

koros-e-ta
殺せた
Was able to kill.
koros-e-nai
殺せない
Not able to kill.