的 / 得 / 地
de / dé / dì
Structural particle "de"
Three characters, one sound: 的 comes before nouns, 得 after verbs, 地 before verbs.
的 + Noun · Verb + 得 · Adj. + 地 + Verb
How it works
红色的车 — 的 attaches a description to a noun. 跑得很快 — 得 attaches an assessment to a verb. 慢慢地走 — 地 attaches a manner to a verb.
All three are pronounced de, which is why native speakers themselves mix them up in writing. The positions never overlap, so the rule is mechanical: what's on the right decides.
得 is also dé (to obtain) and děi (must) in other contexts; as a particle it's always de.
Examples
hóng sè de chē
红色的车
a red car
tā pǎo dé hěn kuài
他跑得很快。
He runs fast.
tā màn màn dì zǒu
他慢慢地走。
He walks slowly.