The one-line test for each

Forget the meanings for a moment and look only at what sits on either side of the particle. Position alone decides which character you need.

• 的 glues a modifier to a NOUN that follows it. Something + 的 + noun.
• 得 hangs a complement off a VERB or adjective that precedes it. Verb + 得 + complement.
• 地 turns a description into the MANNER of a verb that follows it. Description + 地 + verb.

So 的 points forward to a noun, 得 points back to a verb, and 地 points forward to a verb. That is the whole system. Everything below is detail.

的: modifier plus noun

的 is the attributive particle. Whatever sits in front of it is describing the noun that comes after. The thing in front can be a pronoun, an adjective, or a whole clause - it makes no difference, the noun is always next.

• 我的书 (wǒ de shū) - my book (pronoun describes noun)
• 红的车 (hóng de chē) - the red car (adjective describes noun)
• 跑的人 (pǎo de rén) - the person who is running (verb/clause describes noun)
• 老师的话 (lǎo shī de huà) - the teacher's words (noun describes noun)

The test: if a noun follows, you want 的. This is the possessive 我的书 and the general modifier marker rolled into one character. The basic possession side of this is covered in possession with 的; the clause-before-noun side, where a whole sentence sits in front of 的, gets its own treatment in relative clauses with 的.

得: verb plus complement

得 attaches a complement to the verb or adjective in front of it. The complement tells you how well, how much, or whether the action can be completed. The verb comes first, 得 second, the descriptor last.

• 跑得快 (pǎo de kuài) - runs fast (manner: how the running goes)
• 说得很好 (shuō de hěn hǎo) - speaks very well
• 高兴得跳起来 (gāo xìng de tiào qǐ lái) - so happy he jumped up (degree: how happy)
• 看得见 (kàn de jiàn) - can see (potential: whether seeing succeeds)

The test: if the de sits after a verb and pulls a descriptor or result behind it, you want 得. This is the manner complement (跑得快), the degree complement (高兴得跳起来 - happy to the point of jumping) and the potential complement (看得见 - able to see) all using the same particle. The full mechanics of these live in complements and potential complements; here all you need is the rule that 得 follows a verb.

地: manner plus verb

地 turns an adjective or descriptive phrase into an adverbial, marking the MANNER in which the following verb is performed. The description comes first, 地 second, the verb last.

• 慢慢地走 (màn màn de zǒu) - walks slowly (slowly colours the walking)
• 高兴地说 (gāo xìng de shuō) - says happily (happily colours the saying)
• 认真地学习 (rèn zhēn de xué xí) - studies seriously
• 大声地笑 (dà shēng de xiào) - laughs loudly

The test: if the de sits before a verb and the thing in front is describing how that verb is done, you want 地. The description always comes before the action it modifies, exactly the reverse of where 得 puts things.

The two errors that catch everyone

Error 1: writing 的 for all three

Because all three sound identical, the lazy default is to write 的 everywhere. It is wrong in two of the three cases, and it is the single most common slip in learner (and native) writing.

• ✗ 他跑的快 - WRONG. A verb (跑) is in front, so this needs 得: 他跑得快 (tā pǎo de kuài) - he runs fast.
• ✗ 慢慢的走 - WRONG. A verb (走) follows, so this needs 地: 慢慢地走 (màn màn de zǒu) - walk slowly.
• 我的书 - CORRECT. A noun (书) follows, so 的 is right: my book.

Run the position test every time and the default disappears.

Error 2: confusing 得 and 地 because both describe manner

This is the subtle one. Both 得 and 地 can attach a word like 'fast' or 'happy' to an action, so learners treat them as interchangeable. They are not. The difference is which side of the verb the descriptor sits on.

• 地 puts the description BEFORE the verb: 高兴地说 (gāo xìng de shuō) - says happily. You are about to describe an action and you announce the manner first.
• 得 puts the description AFTER the verb: 说得高兴 (shuō de gāo xìng) - talks away happily / talks and is happy doing it. The action happens, then you comment on how it went.

Same three words, different particle, different timing. 地 sets up the manner in advance (the adverb of English: 'he spoke happily'). 得 reports on the result or quality after the fact (closer to 'he spoke, and well / happily he did'). When the descriptor comes first, it is 地; when it trails the verb, it is 得.

Minimal pairs side by side

The cleanest way to feel the system is to hold the same vocabulary still and move only the particle.

Three jobs, three characters, one sound. The vocabulary is nearly identical down each column; only the particle and its position change.

Where 地 can be dropped

地 is the most droppable of the three. With certain adverbials it is optional, and with a few it is normally left out altogether.

• Reduplicated adjectives often keep 地 but can drop it in casual speech: 慢慢(地)走 (màn màn (de) zǒu) - walk slowly.
• Two-syllable manner adverbs that have become fixed usually drop it: 努力学习 (nǔ lì xué xí) - study hard, rather than the heavier 努力地学习.
• Set adverbs of degree and frequency never take 地 at all: 很 (very), 都 (all), 也 (also), 常常 (often) attach straight to the verb with no particle - 常常去 (cháng cháng qù), often goes, never 常常地去.

By contrast, 的 before a noun and 得 before a verb-complement are structural and cannot be dropped: 我书 and 跑快 are both wrong. So when in doubt about whether the de is droppable, the answer is only ever yes for 地.

What to drill

1. Use the position test, not the meaning. Noun after = 的; complement after a verb = 得; verb after a description = 地. The character is decided by the slot.
2. Stop defaulting to 的. It is right only when a noun follows. A verb in front needs 得; a verb behind needs 地.
3. Drill the 得 / 地 pair on manner. Description before the verb is 地 (高兴地说); description after the verb is 得 (说得高兴). Same words, opposite order.
4. Remember 地 is the droppable one. Reduplicated and fixed adverbs can lose it; 的 and 得 are structural and stay.
5. Read the minimal pairs aloud. Hearing 高兴地说 / 说得高兴 / 高兴的人 back to back trains the slot, not the sound.

For the attributive 的 in detail, see possession with 的 and relative clauses with 的. For the 得 complement mechanics, see complements. For how descriptive words behave as predicates in the first place, see adjectives as stative verbs.