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Any difference between the 3 for conditional? Shinai-to can also be just nai-to for verbs like taberu.

Is the difference that the last 2 only can be used at the start of the sentence whhile shinai-to can only be used at the end (unless Sou is added to become "Sou Shina-to")?

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さもないと has a very restricted and loaded usage, and not technically a conditional. It's only used in situations such as "Otherwise / On failure, [here come some bad consequences]".

しないと and でないと are respectively...:

  • しないと = する ("do") + negative + ("if/once...") "if (something) doesn't do"
  • でないと = だ/である (copula) + negative + ("if/once...") "if (something) is not"

But as you said, some phrases starting from a copula are allowed to be used in the beginning of a sentence as conjunctions, as if the entire previous context connects to them: だが, だけど, だから, でも etc.

As such, でないと (and its colloquial form じゃないと) can stand at the beginning of a sentence to mean "if it is not the case" or "otherwise", but しないと just means what it would.

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