ichidan

Ichidan, or 一段動詞, is a class of Japanese verbs also known as ru-verbs. They are characterized by conjugating mostly by dropping the final る from the dictionary form and attaching endings to the remaining stem. The name reflects the idea that their conjugation involves a single, uniform set of endings, in contrast with godan verbs, which change more across forms.

Most ichidan verbs end in either -eru or -iru in the dictionary form, such as 食べる taberu “to eat,” 見る miru “to see,” and 借りる kariru “to borrow.” However, not every -eru or -iru verb is ichidan; some are godan (for example, 走る hashiru “to run”). Irregular verbs such as する suru and 来る kuru do not belong to the ichidan class.

Conjugation works by removing the final る and adding endings to the same stem. For example, 食べる becomes 食べます (polite), 食べない (negative), 食べて (te-form), 食べた (past), and 食べられる (passive or potential in many usages). In general, the stem remains constant across most forms, which distinguishes ichidan patterns from godan patterns where the stem vowel changes.

In summary, ichidan verbs form a key, well-defined group in Japanese verb conjugation, characterized by a uniform stem-based pattern and the frequent endings -る, -ない, -ます, -て, and -た, with irregulars like する and 来る outside this class. They are commonly referred to as ru-verbs and are contrasted with godan verbs in grammar descriptions.