FIRST REVISION: First three lines of a 700 word firm flash fiction titled A DAGGER TO THE BACK.
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No one in the dojang was surprised when Kim Seung-Tae proposed to the southeastern girl, Nalisa. The two of them spent countless hours together, obsessed with fulfilling the Master’s mantra: defeat any opponent by out training them. Long after the sword school closed every evening, the two remained, executing complicated maneuvers on the polished boards of the empty hall.
I haven't properly clocked your initial post, so these are just my thoughts on the revision.
Characters in the story might not be surprised by this proposal, but I am. The way you go on to describe the protagonists, they really come across as "married to the cause" types. The obsession to the Master's mantra, the long hours of practise... it doesn't sound like there's much room for romance in there, which makes makes the marriage come across as (to me) one of pure convenience, or contrived for the sake of the story. Looking at the proposal from the other side of it, having studied the sword for almost a decade now, I can't imagine the protagonists having any sort of success at the art if they're distracted by romance.
I might also cut the 'polished boards' and replace them with 'wooden boards'. Reason being, I've seen how vicious a polished floor can be... and the sight of one particular guy sliding on his ass across the
dōjō (twice!) is what sticks in my mind when I read polished boards.
Although the writing's technically sound, the opening lines here don't have enough of a hook to pique my interest and encourage me to read further. However, that's not necessarily a bad thing, as the writing's solid and the characters established. You don't have to entice your reader to continue with a whole novel, just a few hundred words. I think for this story, the opening could work fine.