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Why does “する” become “すん” in casual Japanese?

Japanese casual speech loves to compress. “するんだよ” (“[you’re] doing it”) often collapses into “すんだよ,” dropping the る and fusing the explanatory んだ straight onto す. It’s the same shortcut behind “何するの?” becoming “何すんの?” — a hallmark of quick, unguarded speech between people who know each other well, parents and kids included.

In the clip, a toddler mimics her dad’s dance moves, and he protests, laughing, “なんでまねすんだよ” (“why are you copying me?!”) — from 真似する (“to imitate”) + するんだよ → すんだよ. The same pattern shows up everywhere in casual speech and anime: 何やってんの (from やっているの), 行くんだ, etc. Textbooks teach the full form するんだよ; native speech almost never bothers with it.