怎麼（一）回事grammar point · tier 1 · 怎麼（一）回事 — what's going on, what's the story behind this
Fixed question that bundles a whole situation into one matter (一回事) and asks for its identity or explanation: what's going on, what's the story with this.
字源 FORM what the parts do
怎 carries a heart (心) underneath that does the wondering, with 乍 lent for a sound ground down to zěn. 麼 is on loan for sound only, worn to a light me; neither the thread (幺) nor 麻 spells anything here. 回 draws a swirl, one ring closing inside another; one full circuit, and so one count of a matter. 事 is a hand gripping a tally-staff, the affair one is charged with. 回 counts that affair as a single whole, 一回事, and 怎麼 asks which kind of affair it is.
故事 STORY a scene to remember it by
A tangle of doings gets wound into one bundle, and the wondering heart turns it over: which single thing is this, all told?
字源記憶法
框 · Frame
[（是）怎麼（一）回事]？ / 知道是怎麼（一）回事 （怎麼（一）回事 takes the seat of one whole matter; 一 is optional, the bundle stays single either way）
觸 · Trigger
A whole situation sits in front of you unexplained, and you want it accounted for as one matter: what is this, taken as a whole.
序 · The move
1Treat the whole situation as one packaged matter, 一回事, not a single action or a single thing.Is the unknown the identity of the whole affair, rather than the way one act is done or the name of one object?
2Front it with 怎麼 and seat it as that one matter: 怎麼（一）回事, 一 optional.Does it ask which kind of affair this is, not bare "how" and not "what came over a person"?
3Read it back; the answer would explain the situation as a whole, an account, not report a method or a reason for a fact already stated.Would the reply lay out what the matter is, rather than tell you how to do something or why a known fact holds?
例 · Examples
1聽說I heard that姐姐older sister跟with室友roommate吵架to quarrel了(change of state)，你you知道to know是is / be怎麼一回事what's going on (what the whole matter is)嗎(yes-no question)？
I heard my sister had a fight with her roommate — do you know what's going on?
怎麼（一）回事 packages the whole situation as one matter and asks for its account, the story behind it; 怎麼了 reacts to a noticed change in someone or something and asks what has come over it. Reach for 回事 when you want the whole affair explained, 怎麼了 when something is suddenly off.
怎麼（一）回事 holds the seat of one whole matter and asks what it is; bare 怎麼 sits before a verb and asks the way one action is done. The 回事 turns "how" into "what is this affair."
怎麼（一）回事 asks for the explanation of a whole situation; 為什麼 asks the single reason behind a fact already stated. Swap by whether you want the matter laid out whole or one cause named.
✗ 這是什麼回事？（thing-word 什麼 where the manner-into-matter 怎麼 belongs）→ ✓ 這是怎麼回事？
✗ 這是怎麼一個事？（the matter is counted with 回, not 個, in this fixed phrase）→ ✓ 這是怎麼一回事？
✗ 他怎麼回事了？（forcing the change-marker 了 onto the whole-matter question）→ ✓ 他是怎麼回事？ / 他怎麼了？
English says "what's going on / what's the story" with "what," so learners reach for 什麼; Chinese carries it on 怎麼 (how) plus 回事 (one whole matter counted by a circuit). The 一 is optional and the matter is counted with 回, not 個 or 件.