Japanese godan and ichidan verbs

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.

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:

• 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:

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.
   
   

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 〜いる (-iru) or 〜える (-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 〜いる (-iru) or 〜える (-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 〜いる (-iru) or 〜える (-eru) are pentagrade verbs.

This includes verbs that rhyme with 〜ある (-aru), 〜うる (-uru) and 〜おる (-oru), which are pentagrade verbs.

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

248 of the 419 〜いる (-iru) verbs [ca. 60%] listed in JMdict are pentagrade verbs.

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

2886 of the 3013 〜える (-eru) verbs [ca. 95%] listed in JMdict are monograde verbs.

Kana and kanji based heuristics for 〜いる (-iru) and 〜える (-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)