The Japanese language has two main types of verbs which are referred to as and .
Verb groups
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 (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 than monograde verbs.
Sometimes categorization is expanded to include a third category of irregular verbs—which most notably include the verbs and . Classical Japanese had more verb groups, such as and , which are archaic in Modern Japanese.
Terminology
Within the terms and , the numbers and correspond with the number of rows that a verb stem (or inflectional suffix) can span in the gojūon kana table. This is best visualized by comparing various verb conjugations to an extracted column of the gojūon table:
In the table above, the verb uses kana from all 5 rows of the gojūon table in its inflectional suffix—, , , and —amongst its conjugations. Thus, it is classified as a "class-5" (or more formally "pentagrade") verb. Meanwhile, the verbs and each use kana from only 1 row of the gojūon table in their verb-stem's suffix— and respectively. Thus, they are classified as a "class-1" (or more formally "monograde") verbs.
Advanced terminology
As monograde verbs only fall into the or rows, they can be further classified into the and subgroups respectively. This is due to being above in the vowel ordering. In full terminology, the gojūon column name of the verb stem's suffix becomes a prefix of these subgroups. For example, the monograde verb has its verb stem in the upper row of the and is formally classified as a verb; meanwhile, the monograde verb has its verb stem in the lower row of the 'ma' column and is formally classified as a verb. Pentagrade verbs are also formally classified into subgroups, but instead use the column name of the verb's inflectional suffix. For example, the pentagrade verb is in the 'ma' column, so it is formally classified as a verb.
Japanese language education
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 " / " terminology.
Consonant and vowel nomenclature
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: is a consonant, then it is a consonant stem verb (pentagrade verb) is a vowel, then it is a vowel stem verb (monograde verb)
There are several flaws with the consonant and vowel nomenclature: When pentagrade verbs end with , the verb's invariant stem always ends with a vowel, yet is still classified as having a consonant stem. For example, has the vowel "a" as the invariant suffix, yet it is still categorized as a "consonant stem verb". In these cases, this apparent contradiction is resolved by realizing that the verb's invariant stem ends in the consonant w. The w is normally suppressed, but surfaces in the negative form, as seen in . Traditionally these verbs ended in -hu, which is still seen on occasion in historical kana usage, and thus unambiguously ended in h.
Verb classification
Classifying verbs is simple in theory: Take the verb in its plain, negative form. The result will be: verb-stem + If the last character of the verb-stem (ignoring the ""): rhymes with , then it is a pentagrade verb rhymes with or , then it is an monograde verb
This classification system works for all Japanese verbs, with three exceptions: is a pentagrade verb, and both and are instead classified as irregular verbs.
Dot notation
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 may be written as 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).