其中grammar point · tier 1 · 其中 — among them, of which (reaching into a named group)
· qízhōng
Names a part inside a group already on the table: of that set, this one.
字源 FORM what the parts do
其 is borrowed for the pointing job — a winnowing basket lent out for the sound, here doing the work of that group just named. 中 plants a pole inside a bounded space: within. Together they reach back into the set and point at what sits inside it.
故事 STORY a scene to remember it by
A group already stands in the room; a finger goes in among them and lands on one.
字源自撰記憶法
框 · Frame
[group named] ，其中 [one/some] [comment]
觸 · Trigger
A group is already in view and you want to single out a member of it.
序 · The move
1name the whole group firstis the set already on the table for 其 to point back at?
2其中 to reach inside itam I picking from that set, not introducing a new one?
3name the member: 其中 + number/measure or 之一is the member a part of the group, never the whole?
4make the comment about that member
例 · Examples
1他們they兩two兄弟brothers，其中of them / among them一個one適合is suited to做生意do business，一個適合教書teach。
The two of them are brothers; of the pair, one is suited to business, the other to teaching.
其中 points back at a group already named (其 = that set); 當中 sits right after a noun as a postposition, 在 N 當中, marking the midst of N without the back-reference.
其中 picks members from inside the set; 其他 picks the rest left outside it. One reaches in, one points away.
no group named first: 其中一個很高 → 他們三個，其中一個很高
其中 standing for the whole group: 我喜歡其中 → 我喜歡其中一個
其中 introducing a new set from scratch: 其中很多學生遲到 → 班上有三十個學生，其中很多遲到
English 'one of them' lets the group sit anywhere or stay implied; 其中 needs the set spelled out first, so learners drop the antecedent and leave 其 pointing at nothing.