The Japanese language has two main types of verbs: godan verbs, or , and ichidan verbs, or .
Categories are important when conjugating Japanese verbs, since conjugation patterns vary according to the verb's category. For example, and belong to different verb categories (quinquegrade and unigrade, respectively) and therefore follow different conjugation patterns. Most Japanese verbs are allocated into two categories:
Statistically, there are about twice as many quinquegrade verbs than unigrade verbs.
Classical Japanese had more verb groups, such as and , which are archaic in Modern Japanese.
The word grade in quinquegrade and unigrade is translated from . In grammar, dan is a synonym for and opposite to . The translations for dan/retsu and gyà  vary, either of them can be translated as "row" or "column" depending on how the gojà «on table is laid out, but the distinction is simply that gyà  is named after consonants, as , while dan/retsu is named after vowels, as in or . The consists the kana , , , , , which differ only by vowels; while the consists of the kana , , , , , , , , , , , , , , , which differ only by consonants.
The class consists of verbs whose inflection forms make use of all five grades, or five vowels. For example, the inflection forms of the verb are /, , , and . These verbs developed from the earlier class, after a historical sound change that turned such forms like into and resulted in an additional vowel (see Late Middle Japanese).
The class consists of verbs that occupy only one grade, or one vowel. Dictionaries may further divide this class into ) if the vowel is the "upper" i, and ) if the vowel is the "lower" e. The verb , whose inflection forms are , , and /, is an example of an "upper unigrade" verb, and the verb is a "lower unigrade" verb. Some unigrade verbs evolved from earlier forms of bigrade verbs ( (sometimes also known as ) and ).
Dictionaries often list ancestral forms of modern verbs as well as their classes. Thus, the entry for kaku may include a note like , which means "verb, ka-row, quinquegrade, formerly quadrigrade"); while the entry for may include , which means "verb, ka-row, lower unigrade, lower bigrade equivalent in Classical Japanese is ".
Note that the choices of prefixes in these English terms by some authors are rather inconsistent: while ' and ' are Greek, ', ', ' and ' are Latin (see ). The word ' is actually Latin, not Greek in origin. While many authors use bigrade and quadrigrade consistently, they also use any combination of unigrade, monograde, quinquegrade, quinquigrade and pentagrade. Some dispense with altogether and prefer only even for modern Japanese (see Godan vs yodan below). Plain English alternatives to "unigrade", "bigrade", "quadrigrade" and "quinquegrade" include "one-grade", "two-grade", "four-grade" and "five-grade"; "one row", "two row" and "four row"; "one-step", "two-step", "four-step" and "five-step"; or "one-vowel", "two-vowel" and "four-vowel".
Some Western analyses refer to "quinquegrade" verbs as "consonant-stem" verbs. Such analyses may represent the root form of the verb as kak-, emphasizing the unchanging consonant k. "Unigrade" verbs are then referred to as "vowel-stem" verbs, for example analyzed as mi-.
Historically, the so-called verbs and were sometimes known as , given that their forms contain three out of the five vowels of Japanese.
Here is a visualization that compares various verb conjugations to an extracted column of the gojà «on table.
In the table above, the verb uses kana from all 5 rows of the gojà «on table in its inflectional suffixâÂÂ, , , and âÂÂamongst its conjugations. Thus, it is classified as a "class-5" (or more formally "quinquegrade") verb. Meanwhile, the verbs and each use kana from only 1 row of the gojà «on table in their verb-stem's suffixâ and respectively. Thus, they are classified as a "class-1" (or more formally "unigrade") verbs.
All modern godan verbs are derived from historical yodan verbs. The distinction between these two classes relies solely on the interaction between the and the auxiliary (historically, ). Consider the verb :
The shift of vowels from au to à  was regular and expansive during Late Middle Japanese, and it practically introduced an additional to the inflectional forms of yodan verbs:
The term is a fairly modern coinage. During the time when modern kana usage was being adopted to write in place of historical kana usage, one of the changes concerned how such a form as kakà  should be spelt. The modern spelling 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 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:
Moreover, the auxiliary -ta and the particle -te also notably alter the ren'yà Âkei:
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 , 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.
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 "quinquegrade" and "unigrade" verbs.
In literature adopting the "Group I / II / III" terminology, the terms (I), (II) or (III) may be notated beside verbs. Similarly, (ãÂÂ) or (ãÂÂ) may be notated beside verbs in literature adopting the " / " 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:
This classification system works for all Japanese verbs, with three exceptions: is a quinquegrade verb, and both and are instead classified as irregular verbs.
In some Japanese dictionaries, the readings of conjugable words may have the stem and the inflectional suffix separated by a dot (ã»). For example, the adjective may be written as to separate the static prefix from the dynamic suffix.
This system also describes the verb group classification: in quinquegrade verbs, the dot is placed before the last kana; in unigrade verbs, the dot is placed before the last 2 kana (except for 2-kana unigrade verbs, which have no dot).
However, regardless of the dot's position, the inflectional suffix is always the last kana of any unigrade 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 quinquegrade verbs ending with âÂÂspecifically, when quinquegrade verbs rhyme with or . Therefore, when a unigrade 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 quinquegrade verbs from unigrade verbs when they rhyme with or . The following heuristics aim to improve the accuracy of naive classification:
Kana and kanji based heuristics for and verbs:
There are many quinquegrade verbs which may be mistaken for being unigrade verbs in some cases . On the surface, this may seem like a problem that only affects conjugation patterns, since quinquegrade verbs and unigrade 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 quinquegrade verbs that resemble unigrade verbs, it is impractical to create or memorize an exhaustive list of words.