A place-phrase (在 + place-word) sits before 的 and turns into a modifier picking out the noun behind it
框 · Frame
(在) [place-word] 的 [noun]
觸 · Trigger
You want to point at a thing by where it sits, then keep talking about the thing.
序 · The move
1Build the place-phrase first: (在) + the place-word.Does it answer 'where', not 'who owns it'?
2Add 的 to clamp the place-phrase onto the noun.Is 的 between the place and the thing, with the thing last?
3The noun comes after 的; the whole bundle is now one thing you can keep using.Could this bundle be the subject of a further remark? If so it is a modifier, not a finished sentence.
例 · Examples
1在桌上on the table的(modifier link)電腦computer是is我弟弟的my younger brother's。
The computer on the table is my younger brother's.
界 · Boundary
locative 在 (電腦在桌上)
(在)PW的N names the thing by its place, leaving you mid-noun to say more (在桌上的電腦…); 電腦在桌上 is a finished sentence stating where it is. Modifier before a noun vs whole predicate — the 的 is the tell: with 的 the place is a label, without it 在 is the verb.
possessive 的 (弟弟的電腦)
Same linker, different slot-filler: an owner before 的 gives whose it is (弟弟的電腦); a place-phrase before 的 gives where it is (在桌上的電腦).
電腦在桌上的 → 在桌上的電腦 (the place-phrase + 的 go before the noun, not trailing after it)
在桌上電腦 → 在桌上的電腦 (a place-phrase modifier needs 的 to clamp onto the noun)
桌上的電腦在 → 在桌上的電腦 (在 heads the place inside the modifier; it does not dangle at the end)
English builds this as a relative clause after the noun ('the computer that is on the table'); Mandarin puts the whole place-phrase + 的 in front of the noun, so the order reverses.