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This question already has an answer here:

どうしたんだ vs どうしたんだい

そうか vs そうかい

This kind of thing. Is it a formality difference? Is it dialectical/Regional? Age related?

marked as duplicate by broccoli forest, Blavius, ajsmart, Community Oct 7 at 1:57

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The i at the end is a shortened form of yo Sou kai = sou ka yo. Doushita ndai = doushita nda yo.

Its slang.

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    Interesting theory; do you have any source on that? – Igor Skochinsky Oct 2 at 19:05
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    sljfaq.org/afaq/kai-question-particle.html. in Wikipedia tho, kai is its own particle. But the link suggests its origin came from the ka-yo particles – skekTek Oct 2 at 19:15
  • That theory doesn't have consensus, and apart from etymology, it's not interchangeable to current "yo". – user4092 Oct 2 at 22:57
  • Yeah, this answer seems to imply that hey can be used interchangeably. – Leebo Oct 3 at 0:10

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