V下grammar point · tier 1 · acquisitive 下: action pulls the thing down into one's keeping (V+下)
· xià
Figurative: 下 fixed after a verb of getting or fixing marks that the thing is brought down into one's own possession or record and held there — bought, kept, written, taken in.
Bare directional V下 lowers a body to rest you can watch (坐下, 放下 — something physically comes down). This V下 lowers nothing physical; it draws a thing into your keeping (買下, 記下). One settles down to the ground, the other settles into possession.
A resultative names the outcome state of the verb (買到 — succeeded in buying, 寫完 — finished writing). This 下 names that the thing was secured and kept (買下 — bought and now held, 寫下 — written down and on record). One says it succeeded or ended, the other says it came into your hands and stayed.
V起來3 gathers a thing and stows it out of the way (收起來 — packed away). This 下 draws a thing down into your keeping, on view or on record (留下, 記下). One puts away, the other takes in and holds.
他下買這棟房子 ✗ → 他買下這棟房子 ✓ (下 follows the verb, never leads it)
他坐下這棟房子 ✗ → 他買下這棟房子 ✓ (坐下 lowers a body to rest; only a verb of acquiring takes this securing 下)
他買下到這棟房子 ✗ → 他買下這棟房子 ✓ (下 attaches to the verb directly; the thing secured follows, with no 到 between)
English 'buy UP', 'write DOWN', 'take DOWN' tack a free particle after the verb that drifts and can take its own slot; Chinese fixes 下 hard against the verb, and the 'down' here means into your keeping, not toward the floor.