Tails a verb to report the action succeeded — the thing was reached, found, gotten, perceived.
框 · Frame
[verb] 到 (+ obj) (了)
觸 · Trigger
You searched and the thing turned up; you listened and the sound came through — the verb connected with what it aimed at.
序 · The move
1Say the verb, then weld 到 directly onto its back.Is 到 fused to the verb (找到, 聽到), not floated off as a separate word?
2Name the thing the verb landed on after 到; add 了 if the success is complete.Is this the target the action got, rather than a clock time or boundary it ran up to?
3Read it as 'did the verb, and it connected'.Does it report a hit landed once, not how far an ongoing action stretched?
例 · Examples
1他找look for / search到那家超市that supermarket (the target reached)了marks the success as done。
V到3 reports a target struck in one stroke — 找到, 聽到, 買到, you got it. V到1 names a boundary an ongoing action runs up to and halts at — 上到五點, the teaching lasted until five. V到3 lands on a thing; V到1 measures how far it ran.
V到3 is one resultative outcome — the specific result is reached / attained. 結果補語 is the whole family of verb+outcome (看懂, 寫完, 洗乾淨); 到 is the member that means the act connected with its target.
V到3 says the result happened — 找到了, found it. 可能補語 inserts 得/不 to say whether it could happen at all — 找得到 (can find), 找不到 (can't find). One reports the hit; the other rates the chance of it.
我找那家超市了。 → 我找到那家超市了。 (找 alone is only the searching; 到 is what reports it succeeded)
我聽了那個聲音，可是沒清楚。 → 我沒聽到那個聲音。 (the failed-attainment is 沒 + V到, not a loose negative)
他找了到那家超市。 → 他找到那家超市了。 (了 marks the whole success at the end, not wedged between verb and 到)
English splits the try from the success into two verbs — 'looked for' versus 'found', 'listened' versus 'heard'. Chinese keeps the same verb and tacks 到 on to flip trying into getting (找 → 找到, 聽 → 聽到); learners drop the 到 and leave only the attempt.