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I think it might be to just differ between words, but why isn't 殺す こらす, 下ろす 下らす, etc. Is it phonetic change? Is that the reason why 足りる isn't 足るる。For passives, 戻る isn't もだる.

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    The passive of 殺す is 殺される, and the causative is 殺させる・殺さす. These conjugations in no way change the ろ. They only affect the final mora of the word, so I'm not sure what you're asking. Are you getting confused on the conjugations?
    – istrasci
    Aug 28, 2023 at 16:11
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    I don't understand your question. For example, are you asking why the causitive form of 殺す isn't こらす? On what basis do you think it ought to be? Where does お vs あ in your title come into it? Aug 28, 2023 at 16:12
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    "but why isn't" - why should it be? Step by step, what is your reasoning to get that result? Aug 28, 2023 at 16:31
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    If you are thinking 殺す and 下ろす are causative forms of, respectively, こる and おる, they are not. I have no idea what you are thinking 足りる and 戻る are, or why you think they should be たるる(with /u/) and もだる (with /a/). Passive forms have /e/ there.
    – aguijonazo
    Aug 28, 2023 at 16:54
  • Let us continue this discussion in chat.
    – Star Peep
    Aug 28, 2023 at 19:19

1 Answer 1

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Looks like you've mixed transitive/intransitive pairs, plain/causative pairs and even old/modern pairs.

  • Causative forms follow certain grammar rules. It always ends with either -(さ)せる or -す. It can be used with almost any verb, including transitive verbs. Just as the past form of a verb is not listed in dictionaries, causative forms are not listed in dictionaries.
  • Transitive verbs are words on their own, and have their own entries in dictionaries. You basically cannot predict a transitive verb from an intransitive verb, although there are some useful patterns. Many transitive verbs (食べる, 殺す) do not have an intransitive counterpart in the first place. While some transitive verbs, such as 動かす and 乗せる, might originate etymologically from causative expressions, this does not mean people today consider them to be causative forms.
  • Pairs like 足る/足りる, 流る/流れる, 燃ゆ/燃える are old/modern pairs. They are of course etymologically related, but have nothing to do with transitivity or causative rules.

In summary, your question is essentially about the etymology of transitive verbs. However, like the etymology of many other words, it is fundamentally unpredictable. You need to memorize each transitive verb. This is a different issue from the grammatical rules of causative forms.

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