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).

Verb groups

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:^[1]

1. Pentagrade verbs (五段動詞, godan-dōshi, literally: "Class-5 verbs")
2. Monograde verbs (一段動詞, ichidan-dōshi, literally: "Class-1 verbs")

Statistically, there are far more pentagrade verbs^[2] than monograde verbs.^[3]

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, "Class-2 verbs")^[4] and quadrigrade verbs (四段動詞, yodan-dōshi, "Class-4 verbs"),^[5][6] which are archaic in Modern Japanese.

Terminology

Within the terms "pentagrade verbs" (五段動詞) and "monograde verbs" (一段動詞), the numbers go (五, 5) and ichi (一, 1) 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 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. (See also Japanese verb conjugations.)

Advanced terminology

As monograde verbs only fall into the 〜い (-i) or 〜え (-e) rows, they can be further classified into the 上一段 (kami ichidan, "upper monograde") and 下一段 (shimo ichidan, "lower monograde") subgroups respectively. This is due to い (i) being above え (e) in the あいうえお (a-i-u-e-o) 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 見る (miru) has its verb stem in the upper row of the 'ma' column (マ行, magyō) and is formally classified as a マ行上一段活用 (ma-row upper monograde conjugation) verb; meanwhile, the monograde verb 止める (tomeru) has its verb stem in the lower row of the 'ma' column and is formally classified as a マ行下一段活用 (ma-row lower monograde conjugation) 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 読む (yomu) is in the 'ma' column, so it is formally classified as a マ行五段活用 (ma-row pentagrade conjugation) 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 "う-verbs / る-verbs" 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 criticisms of the consonant and vowel nomenclature:

1. When pentagrade verbs end with "う" (u), the verb's invariant stem always ends with a vowel, yet is still classified as having a consonant stem. For example, 買う (kau, to buy) 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 買わない (kawanai, to not buy). Traditionally these verbs ended in -hu, which is still seen on occasion in historical kana usage, and thus unambiguously ended in h.
   
   
2. When godan verbs end with "つ" (tsu), the verb's invariant stem always ends with an "s" rather than a "t". Since the consonant stem terminology focuses on rōmaji, this could lead to conjugation errors. For example, 待つ (matsu, to wait) in its negative conjugation does not become "待つぁない" (matsanai) as the consonant stem system might have one believe; the correct conjugation is 待たない (matanai, to not wait). The matter is resolved when phonemic notation of "tu" used by Kunrei-shiki romanization is applied instead.
   
   
3. In the case of the past-tense and te forms of conjugation, the 'invariant' stem changes such that the consonant is removed from all godan verbs (except verbs ending in "つ" (tsu) or "す" (su)). This means the defining characteristic of consonant stem verbs cannot be used to define consonant stem verbs for the past-tense or te forms. The true "invariant stem", which is consistent amongst all conjugations, precedes the so-called "invariant consonant".
   
   

Verb classification

Classifying verbs is simple in theory:

1. Take the verb in its plain, negative form. The result will be: verb-stem + ない (nai)
2. If the last character of the verb-stem (ignoring the "ない"):

• rhymes with 〜ぁ (-a), then it is a pentagrade verb
• rhymes with 〜ぃ (-i) or 〜ぇ (-e), then it is an monograde verb

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.^[7]

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 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.

Naive verb classification

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.

Rules of thumb

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:

• There are far more pentagrade verbs^[2] than monograde verbs.^[3]
• Verbs that do not rhyme with 〜ぃる (-iru) or 〜ぇる (-eru) are pentagrade verbs.

This includes verbs that rhyme with 〜ぁる (-aru), 〜ぅる (-uru) and 〜ぉる (-oru), which are pentagrade verbs.

• The majority of verbs that rhyme with 〜ぃる (-iru) are pentagrade verbs.

248 of the 419 〜ぃる (-iru) verbs [ca. 60%] listed in JMdict are pentagrade verbs.^{[citation needed]}

• The majority of verbs that rhyme with 〜ぇる (-eru) are monograde verbs.

2886 of the 3013 〜ぇる (-eru) verbs [ca. 95%] listed in JMdict are monograde verbs.^{[citation needed]}

Kana and kanji based heuristics for 〜ぃる (-iru) and 〜ぇる (-eru) verbs:

• Verbs written entirely in hiragana are pentagrade verbs. For example, びびる (bibiru, to be surprised) and のめる (nomeru, to fall forward) are pentagrade verbs.
• Kanji verbs with 1 okurigana and 3+ syllables are pentagrade verbs. For example, 契る (chi-gi-ru, to pledge) and 嘲る (a-za-ke-ru, to ridicule) are pentagrade verbs.
• Kanji verbs with 2 okurigana are usually monograde verbs. For example, 起きる (okiru, to get up) and 食べる (taberu, to eat) are monograde verbs.
• Kanji verbs with 2 syllables are inconclusive. For example, 切る (ki-ru) and 見る (mi-ru) are both 2-syllable verbs, yet belong to different categories (pentagrade and monograde, respectively)

Pentagrade verbs resembling monograde verbs

There are many pentagrade verbs which may be mistaken for being monograde verbs in some cases (see § Naive verb classification, above). On the surface, this may seem like a problem that only affects conjugation patterns, since pentagrade verbs and monograde verbs conjugate differently (See main article: Japanese verb conjugations). 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 intonations. (See also Japanese pitch accent.) 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.