Struggling with Japanese verbs that don't follow the rules? Learn why these two 'rebels' are the key to natural conversation and how to master them.
These irregular verbs aren't just random mistakes; they are the flexible joints of the language that have been shaped by the human voice over a thousand years to express how reality manifests and affects us.
Demystifying Japanese irregular verb
These two verbs are considered the "grandfathers" of the language, dating back over a thousand years to Old Japanese. Because they are used so frequently in daily life—meaning "to do" and "to come"—they have resisted the natural "smoothing out" process that standardized other verb patterns over time. Their irregularity is a result of being so central to the language that their unique, ancient forms were preserved rather than forced into modern regular boxes.
The verb kuru is unique because its stem changes vowels depending on the form being used, moving through the "K" column of the hiragana chart. In its dictionary form, it uses ku (kuru); in polite or past forms, it shifts to ki (kimasu, kita); and in negative or potential forms, it shifts to ko (konai, korareru). This "musical chairs" with vowels is why it is specifically categorized as a "K-column irregular" verb.
While many learners study them as separate vocabulary words, dekiru is actually the potential form of suru. Historically, suru was so irregular that it did not develop a standard potential conjugation; instead, it borrowed dekiru (which originally meant "to come forth" or "to appear") to express the ability to do something. This is also why dekiru uses the particle ga instead of o, as it describes a state of existence or "manifestation" of ability rather than a direct action.
"Ra-nuki" refers to the practice of dropping the ra syllable from potential verb forms, such as saying koreru instead of korareru. While traditional grammar rules label this as incorrect, many speakers use it to distinguish between the potential form ("can do") and the passive form ("was done to"), which are otherwise identical for certain verbs. Using "ra-nuki" helps clear up ambiguity in casual speech, though the full form is still required in formal or academic settings.
In Japanese, suru is used to describe sensations that "arise" or "manifest" spontaneously rather than actions a person actively performs. For example, instead of "having" a headache or "smelling" a scent, a speaker says the headache or smell is "doing" (zutu ga suru or nioi ga suru). This same logic applies to physical traits, such as "doing blue eyes" (aoi me o shite iru), treating these characteristics as states that are currently manifesting.
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