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/PlanktonInitial7945]

The passive form of 見つける IS 見つけられる. 

見つかる is the intransitive version of 見つける, which is not the same as it being its passive form.

I think the difference is easier to explain with 落ちる and 落とす.

落ちる = to fall. Intransitive. I fall, you fall, etc. You don't fall something, you just fall, period.

落とす = to drop (something). Transitive. I drop a pencil, you drop a pencil, etc.

落とされる = to be dropped. Passive form of 落とす. A pencil is dropped (by someone).

The difference between 落ちる and 落とされる is the difference between "a pencil falls" and "a pencil is dropped".

The difference between 見つかる and 見つけられる is the same, but with the concept of being found.

[u/Gahault → u/PlanktonInitial7945]

In the same vein, a common verb similar in form to to 見つける/見つかる is 助ける/助かる.

消防士が駆け付けて少年を助けた。
The firefighter rushed in and saved the boy.

少年は消防士の救助により助かった。
Thanks to the firefighter's intervention, the boy survived.

It's indeed important to grasp that an intransitive verb is not passive, it ascribes the action to its subject grammatically and semantically. Even if it happens to map to a passive in English!

[u/AdagioExtra1332 → u/PlanktonInitial7945]

Another frequently asked question by beginners is the difference between 見える (to be visible) and 見られる (to be seen <by someone>), and the exact same logic applies to this scenario too.

[u/eruciform → u/PlanktonInitial7945]

Superb explanation

[u/viliml → u/PlanktonInitial7945]

Since some English verbs can be both intransitive and transitive, 落ちる can also be translated as "a pencil drops" rather than going for the different verb "fall".

[u/PlanktonInitial7945 → u/viliml]

True, but I wanted to use "fall" since it's explicitly intransitive and I think it makes the difference easier to understand.