This might be the most interesting word in all of Japanese. The first suggestion you get when you type it into Google is 「違くない 間違い」. It’s widely considered “a mistake” by Japanese people. So why is it SO widely used?
Even jisho.org defines it as a “grammatically incorrect negation of 違う”. Nice prescriptivism jisho 🙄. Why is it considered “incorrect”? Well, 違う is a verb, but 違くない follows the negative conjugation rule for an い-adjective, not a verb. In fact, this paper gives multiple examples of the verb 違う being treated as an adjective in multiple different ways, not just in the form 違くない. 違さ、違ければ、違すぎる and more have all been observed in fairly common use on Twitter.
What’s going on here? How did this happen?
Let’s talk about verb conjugation in Japanese for a sec. Conjugation rules for verbs in Japanese feel really tidy because every possible vowel gets a chance at being connected to the verb stem.
With an example, 手伝う (since it ends in う, same as 違う):
あ Negative: 手伝わない (To not help)
い Nounification: 手伝い (Help)
う Present/future tense: 手伝う (To help)
え Hypothetical: 手伝えば (If I help…)
お Volitional: 手伝おう (Let’s help)
Whatever column the final mora of the base verb is (う for 違う), conjugation just involves moving up or down that column.
Speaking from personal experience, as I acquired Japanese more and more, I stopped thinking about verbs as single words and started thinking about them as stems. Like, 「引く」 isn’t a standalone word. It’s like a pokemon forme change of the verb stem 「引k」. Note the K at the end - since the consonant doesn’t change as the vowel does. I think this is how native Japanese speakers subconsciously think about verbs, even if they don’t realise it.
To phrase it another way, the final vowel sound of a verb isn’t considered important, since it changes whenever you do anything to the verb. What’s more important is what column that last kana belonged to.
Can you see where I’m going with this? 違う ends with う, which is in the same column as い, like an い-adjective. This gives it an adjective-y feeling, more than verbs that don’t end in う. I believe that if 違う didn’t end in う, and it was something like 違す or 違る, this 違くない form would never have come about.
But wait, 違う is the only verb in all of Japanese that has started to do this. If ending in う was the only reason, we’d be seeing other verbs like 笑う and 叶う behaving the same way. So there must be something else.
違う makes sense as an adjective
違う is a stative verb. Stative verbs are verbs that describe the state of a noun. Often, a word that’s a stative verb in one language will be an adjective in another (see: 好き/like; 足りる/enough). Even within the same language, adjectival and verbal forms can coexist (see: 痛む/痛い). It’s easy to see how a stative verb could evolve into an adjective.
On top of this, in Japanese, the grammar for [noun][stative verb] sentences are the same as [noun][adjective] sentences: 「足が痛む・足が痛い」which means that speakers don’t even need to adjust their grammar to start treating it as an adjective.
What seems likely is that people started to think of 違う as an adjective, then eventually began conjugating it like an い-adjective, due to it ending in an あ行 kana.
In that same paper linked above, they note that (regardless of its appearance) if 違う is acting on two nouns at once, it is a verb, but if its acting only on one noun, its an adjective. The reasoning being that verbs assign semantic roles to words in the sentence, while adjectives don’t affect semantic roles at all. Consider the following:
彼の意見は私のと違う
His opinion is different to mine.
That paper seems to imply that when 違う is acting on a noun followed by と, it isn’t conjugated like an adjective. So the following shouldn't sound natural.
I'm not sure if I agree with the paper here. I think the above actually does sound fairly natural, despite it being a case of adjective-y conjugation in a case with two nouns. Which means in every case, 違う is behaving like an adjective.
I believe that 違う isn’t just acting like an adjective in these cases. It IS an adjective! It’s an adjective that has the appearance of a verb!
That said, it is also actually a verb, and can be conjugated as such. It’s just that those conjugations are getting less and less frequent, and less and less natural. Although you wouldn’t use 違くない in a formal piece of writing, I don’t believe you would use 違わない or 違っていない either - you would probably use a different word altogether, like 一致 or 異なっていない. In contrast, conjugating it as if it were an adjective leads to more natural sounding speech. If usage of 違う continues in the direction it’s going, soon “違う is the only い-adjective in Japanese that doesn’t end in い” won’t be a “take”, it’ll just be a fact that all Japanese learners will need to memorise. Hell, they should probably memorise it now.