誓うか必ず戻ると was used when a character asked another to promise to return.

Isn't it supposed to be 戻らないと? Promise that you must come back?

My understanding is "~nai to" is used for "must" where as "~to" is used for
"if/when" you do something.

What meaning the "~to" mean in this sentence?
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2018-06-13T17:16:23.580
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2018-06-13T17:46:14.123
2018-06-13T17:27:04.550
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question
grammar meaning word-choice quotes
Is ~と used the same as ~ないと in this case?
This sentence is inversion and と is used as a particle for quotation. It means
必ず戻ると誓うか.

"~nai to" is used for "must" as you said, but this is rarely used with 誓う. So
誓うか必ず戻らないと(いけないと) is unnatural.
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2018-06-13T17:46:14.123
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answer