Suffixed to a verb to mark the action as ongoing, caught mid-process — the status-label sense of 'in V / under V'.
框 · Frame
[topic] [verb] 中
觸 · Trigger
A process is underway and you want to flag its status as ongoing, label-style, without staging a live doer.
序 · The move
1Name the action as a process that has started but not finished: 調查, 處理, 進行, 開會.Is there a span to sit inside? A one-shot or completed event has no middle to mark.
2Pin 中 directly to the back of that verb, no object between them.中 hangs on the verb itself (調查中), not after an object.
3Read the whole as a status-label of the topic, common in formal or written notices.Does it report a state ('under V'), not narrate a live actor doing it right now? If you need a person mid-act, reach for 在V instead.
例 · Examples
1意外發生的原因the cause of the accident目前at present仍still在調查investigate — the verb中mid-process, the action ongoing。
The cause of the accident is, for now, still under investigation.
界 · Boundary
在V
在V puts a live actor mid-act, narrating what someone is doing now (他在調查). V中 hangs on the verb as a status-label of the topic, no doer needed (案子調查中). Live action versus ongoing-status stamp.
V著1 holds a state open as a continuing condition (門開著). V中 marks a process running through its span toward an end (處理中). Held-still state versus mid-run process.
了1 closes the action at the finish edge, done (調查了). V中 plants it at the center, not yet done (調查中). Completed versus in-progress.
✗ 他在調查中那件案子 → ✓ 那件案子調查中 (V中 is a label on the verb, takes no object after it)
✗ 我吃飯中 → ✓ 我在吃飯 (a live person mid-act wants 在V, not the status-label V中)
✗ 會議開了中 → ✓ 會議進行中 (no 了 inside; V中 marks unfinished, 了 marks finished)
English '-ing' covers both 'he is investigating' and 'under investigation', so learners stretch 在V to cover the status-label sense; V中 is the noticeboard 'in progress' stamp, not the live-action one.