Japanese learners often encounter what textbooks call “adjectival nouns” or keiyōdōshi (形容動詞). While this term is widely accepted, research suggests that “nominal adjective” would be more accurate,
“for these words are really a subclass of adjectives,”
as noted in “A History Of The Japanese Language” by Bjarke Frellesvig.
Where do they come from?
The distinction becomes clear when we look at the historical development of these words. Many of today’s keiyōdōshi trace their origins back to Old Japanese, where they were actually classified as adverbs. Common examples include:
What makes these words particularly interesting is their grammatical behavior. Unlike true adjectives, keiyōdōshi use forms of the regular copula for various grammatical functions. This can be observed in:
Development
The historical development of these words is quite interesting. In Old Japanese, these forms were adverbialized using ni (or to), adnominalized using no, and predicated by combining with existential verbs. A significant shift occurred during the Early Middle Japanese period with the emergence of the inflected copula -> nar-.
The influence of foreign languages, particularly Chinese, played a crucial role in shaping this word class. While there weren’t many Sino-Japanese adjectival nouns initially, the class became increasingly open to loanwords over time. Today, we see this pattern continuing with modern borrowings:
The Adjectival Copula
The distinction between regular and adjectival copula provides the strongest evidence for why keiyōdōshi should be considered a separate class. The regular copula, used with keiyōdōshi, has relatively simple forms (ni, no), while the true adjective shows more complex conjugation patterns including conclusive (ki), exclamatory (kyere), conditional (kyeba), infinitive (ku), and gerund (kute) forms.
This difference in complexity and function becomes particularly apparent when we look at modal and aspectual extensions. Forms like be- (necessitive) kaku be- (must write), rasi- (presumptive) kaku rasi (seems to write), and nar- (evidential) kaku nar- (appears to write) interact differently with these copulas, highlighting their distinct grammatical roles. The adjectival copula’s ability to combine with various verbal suffixes and its more complex inflectional patterns further distinguish it from the simpler regular copula used with keiyōdōshi.
These “inflecting clitics” in Old Japanese follow a finite verb form to create an extended verb syntagm.
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