You stand somewhere and want to say what exists, shows up, or has gone from that spot, the thing being news rather than someone already named.
序 · The move
1Open with the place-word; 在 is not needed once it heads the sentence.Does the sentence begin with where, not who?
2Put the verb of existing, appearing, or vanishing next, wearing 著 for a held state or 了 for one that landed.Is the verb between the place and the thing, not after it?
3Place the thing last, marked as a quantity or indefinite, never a known name.Could the listener already point to this thing? If yes, it does not belong in the end slot.
例 · Examples
1餐桌上on the dining table (place leads)放are set / sit著and stay that way很多新鮮的水果lots of fresh fruit (new, indefinite, comes last)。
存現 ends on a new indefinite thing being introduced onto the stage (桌上放著水果, there is fruit there); topic-comment leads with a known thing set down to be remarked on (那些水果放在桌上, as for that fruit, it is on the table). New thing last against known thing first is the tell, and the order will not reverse: a name already known cannot fill the existential end slot.
存現 puts the place first and the thing last (牆上掛著一張畫, on the wall hangs a painting), staging something new; 在+place puts the thing first and the place last (那張畫在牆上, that painting is on the wall), locating something known. Flip the order and you flip which one is news.
✗ 很多新鮮的水果放著在餐桌上。 → ✓ 餐桌上放著很多新鮮的水果。 (the thing must not lead; the place opens)
✗ 餐桌上那些水果放著。 → ✓ 餐桌上放著很多新鮮的水果。 (a thing already known cannot sit in the existential end slot)
✗ 在餐桌上放著水果。 → ✓ 餐桌上放著水果。 (once the place-word heads the sentence, 在 drops)
English fronts this with dummy 'there' and keeps the thing's slot fixed ('there is fruit on the table'); Mandarin has no 'there', so the place itself fronts and the new thing falls to the end, and learners wrongly lead with the noun or insert 在.