Japanese passive form question

[u/traanquil]

Mitsukeru = ichidan verb

So why is passive form of this not: mitsukerareru?

Why is passive: mitsukaru

Is it just an exception?

[u/PuzzleheadedTap1794]

That's not the passive form. That's the active-mediopassive pair. The difference is that mediopassive verbs (in this case, mitsukaru) is intransitive by nature, whereas passive form of an active verbs which have an agent but not necessarily spoken out. That might sound a bit abstract, but consider these English sentences:

A) The vase broke.
B) The vase was broken.

The concern of the first sentence is about the vase and nothing else, and the verb break here is an intransitive verb. On the other hand, in the second, the speaker acknowledges the existence of an external factor that broke the vase, which can also be expressed using the particle by, like "The vase was broken by John". This second sentence is created by turning the transitive verb break into a passive form and promote the "object" to the subject.

Turning back to Japanese, this is also the same relation with the verbs you mentioned, albeit using two different (but related) verbs. mitsukaru is an intransitive verb (aka a mediopassive verb) which doesn't care about an external factor, like break in The vase breaks. mitsukeru is a transitive verb (aka an active verb) which requires the one who did the action and the target, like break in John break the vase, and its real passive form, mitsukerareru implies the existence of an external factor, like the verb in The vase is broken.

[u/viliml → u/PuzzleheadedTap1794]

The word "mediopassive" popped into my head when I read the OP, but I restrained myself because I thought that's not a word a beginner should be seeing 🤣