The Japanese language has two main types of verbs which are referred to as pentagrade verbs (五段動詞, godan-dōshi) and monograde verbs (一段動詞, ichidan-dōshi).
Categories are important when conjugating Japanese verbs, since conjugation patterns vary according to the verb's category. For example, 切る (kiru) and 見る (miru) belong to different verb categories (pentagrade and monograde, respectively) and therefore follow different conjugation patterns. Most Japanese verbs are allocated into two categories:
Statistically, there are far more pentagrade verbs[3] than monograde verbs.[4]
Sometimes categorization is expanded to include a third category of irregular verbs—which most notably include the verbs する (suru, to do) and 来る (kuru, to come). Classical Japanese had more verb groups, such as bigrade verbs (二段動詞, nidan-dōshi)[5] and quadrigrade verbs (四段動詞, yodan-dōshi),[6][7] which are archaic in Modern Japanese.
The word grade in pentagrade and monograde is translated from 段 (dan). The most familiar use of this Japanese word in English contexts is for ranking in martial arts. In grammar, 段 is a synonym for 烈 (retsu)[8] and opposite to 行 (gyō). The translations for 段/烈 and 行 vary, either of them can be translated as "row" or "column", but the distinction is simply that 行 is named after consonants, as か行 (ka-gyō "ka-row"), while 段/烈 is named after vowels, as in い烈 (i-retsu "i-column") or い段 (i-dan "i-grade"). The あ行 (a-gyō "a-row") consists the kana あ (a), い (i), う (u), え (e), お (o), which differ only by vowels; while the あ段 (a-dan "a-grade") consists of the kana あ (a), か (ka), が (ga), さ (sa), ざ (za), た (ta), だ (da), な (na), は (ha), ば (ba), ぱ (pa), ま (ma), や (ya), ら (ra), わ (wa), which differ only by consonants.
The 五段 (godan "pentagrade" or "quinquigrade") class consists of verbs whose inflection forms make use of all five grades, or five vowels. For example, the inflection forms of the verb 書く (kaku "to write; to draw", ka-row) are 書か (kaka)/書こ (kako), 書き (kaki), 書く (kaku), and 書け (kake). These verbs developed from the earlier 四段 (yodan "quadrigrade") class, after a historical sound change that turned such form as 書かむ (kakamu) into 書こう (kakō) and resulted in an additional vowel (see Late Middle Japanese).
The 一段 (ichidan "monograde") class consists of verbs that occupy only one grade, or one vowel. Dictionaries[8][9][10][11] may further divide this class into 上一段 (kamiichidan "upper monograde"[2]) if the vowel is the "upper" i, and 下一段 (shimoichidan "lower monograde"[2]) if the vowel is the "lower" e. The verb 見る (miru "to look", ma-row), whose inflection forms are 見 (mi), 見る (miru), 見れ (mire) and 見ろ (miro)/見よ (miyo), is an example of an "upper monograde" verb, and the verb 得る (eru "to earn", a-row) is a "lower monograde" verb. Some monograde verbs evolved from earlier forms of bigrade verbs (上二段 kaminidan and 下一段 shimonidan).
Dictionaries[8][9][10][11] often list ancestral forms of modern verbs as well as their classes. Thus, the entry for 書く may include a note like （動カ五［四］）,[8] which means "verb, ka-row, pentagrade, formerly quadrigrade"); while the entry for 受ける (ukeru "to take") may include （動カ下一）[文]カ下二 う・く,[8] which means "verb, ka-row, lower monograde, lower bigrade equivalent in Classical Japanese is 受く".
Note that the choices of prefixes in these English terms by some authors are rather inconsistent: while mono- and penta- are Greek, bi-, quadri- and quinqui- are Latin. The word grade is actually Latin, not Greek in origin. Plain English alternatives to "pentagrade/quinquigrade", "quadrigrade", "bigrade" and "monograde" include "five-grade", "four-grade", "two-grade" and "one-grade",[12] or "four rows", "two rows" and "one row."[13]
Some Western analyses refer to "pentagrade/quinquigrade" verbs as "consonant-stem" verbs. Such analyses may represent the root form of the verb 書く as kak-, emphasizing the unchanging consonant k. "Monograde" verbs are then referred to as "vowel-stem" verbs.[14]
Here is a visualization that compares various verb conjugations to an extracted column of the gojūon table:
In the table above, the verb 読む (yomu, to read) uses kana from all 5 rows of the gojūon table in its inflectional suffix—ま (ma), み (mi), む (mu), め (me) and も (mo)—amongst its conjugations. Thus, it is classified as a "class-5" (or more formally "pentagrade") verb. Meanwhile, the verbs 見る (to see) and 止める (to stop) each use kana from only 1 row of the gojūon table in their verb-stem's suffix—み (mi) and め (me) respectively. Thus, they are classified as a "class-1" (or more formally "monograde") verbs.
All modern godan verbs are derived from historical yodan verbs.[15] The distinction between these two classes relies solely on the interaction between the mizenkei (未然形) and the "tentative" (推量, suiryō) auxiliary u (う) (historically, mu (む)). Consider the verb kaku (書く, "to write"):
The shift of vowels from au to ō was regular and expansive during Late Middle Japanese, and it practically introduced an additional dan (段, in other words, vowel) to the inflectional forms of yodan verbs:
The term godan (五段) is a fairly modern coinage. During the time when modern kana usage was being adopted to write modern Japanese (口語, kōgo, lit. 'spoken language') in place of historical kana usage, one of the changes concerned how such a form as kakō should be spelt. The modern spelling かこう was proposed along with godan as the name for the modernized yodan class. Traditionalist grammarians, on the other hand, would insist on such spelling as かかう to reflect the historical pronunciation kakau, and on the modern pronunciation being inferred from such spelling. Some argued that a single interaction with the auxiliary u did not justify creating an entire new grammatical class, given that the mizenkei does not involve a vowel shift with any other auxiliary:[15]
Moreover, the auxiliary -ta and the particle -te also notably alter the ren'yōkei:
Yet, such alterations are not reflected by either the term yodan or the term godan at all, despite occurring in both these supposedly different inflections (although in classical Japanese (文語, bungo, lit. 'written language'), these alterations in pronunciation must be inferred from the spellings). This means that exceptional interactions with auxiliaries and particles like these ought not to be the basis for naming verb classes.[15]
Obviously, the spelling reform took place and the term godan became mainstream. Historical kana usage is now reserved only for the writing of classical Japanese, and yodan verbs are largely considered a classical Japanese class while godan verbs make up a fundamental part of modern Japanese.
Within Japanese language education, various terminologies are used in lieu of the Japanese nomenclature for "pentagrade" and "monograde" verbs.
In literature adopting the "Group I / II / III" terminology, the terms (I), (II) or (III) may be notated beside verbs. Similarly, (う) or (る) may be notated beside verbs in literature adopting the "う-verbs / る-verbs" terminology.
The terms "consonant stem verbs" and "vowel stem verbs" come from a pattern that emerges from studying the actual structure of the words rather than the written representation. When considering the invariant part of the verb (the verb stem), the final phoneme determines the classification of the verb group. If the verb stem's final phoneme:
There are criticisms of the consonant and vowel nomenclature:
Classifying verbs is simple in theory:
verb-stem + ない (nai)verb-stem (ignoring the "ない"):This classification system works for all Japanese verbs, with three exceptions: ある (aru) is a pentagrade verb, and both しない (shinai) and こない (konai) are instead classified as irregular verbs.[16]
In some Japanese dictionaries, the readings of conjugable words may have the stem and the inflectional suffix separated by a dot (・). For example, the adjective 赤い (akai, red) may be written as あか・い (aka·i) to separate the static prefix from the dynamic suffix.
This system also describes the verb group classification: in pentagrade verbs, the dot is placed before the last kana; in monograde verbs, the dot is placed before the last 2 kana (except for 2-kana monograde verbs, which have no dot).
However, regardless of the dot's position, the inflectional suffix is always the last kana of any monograde verb.
A caveat of accurately classifying verb groups is that you must have pre-existing knowledge of the verb's negative form. In practice, people tend to learn the verb's plain form first. As such, Japanese language educators usually teach strategies for naive verb classification. Whilst such strategies are not comprehensive, they generally remain useful in the context of regular daily conversations that language beginners will likely encounter. Here is one such strategy:
Naive strategies, such as this one, tend to misidentify pentagrade verbs ending with る (ru)—specifically, when pentagrade verbs rhyme with 〜ぃる (-iru) or 〜ぇる (-eru). Therefore, when a monograde verb is concluded from a naive strategy, it is more efficient to confirm the verb's classification in a dictionary. However, there are other rules-of-thumb to more accurately discriminate such verbs.
If a dictionary is unavailable, it becomes difficult to discriminate pentagrade verbs from monograde verbs when they rhyme with 〜ぃる (-iru) or 〜ぇる (-eru). The following heuristics aim to improve the accuracy of naive classification:
Kana and kanji based heuristics for 〜ぃる (-iru) and 〜ぇる (-eru) verbs:
There are many pentagrade verbs which may be mistaken for being monograde verbs in some cases . On the surface, this may seem like a problem that only affects conjugation patterns, since pentagrade verbs and monograde verbs conjugate differently . However there are many verbs that, despite having the same spelling, have different meanings and belong to different verb groups. For example:
When reading verbs such as these, the correct word meaning can be ascertained through the different kanji or accentuation. However, ambiguity is usually removed if the verbs have been conjugated somehow, because different word groups conjugate with slightly varying pronunciations. For example:
Since there are so many pentagrade verbs that resemble monograde verbs, it is impractical to create or memorize an exhaustive list of words.