Third-Tone Sandhi: Why nǐ hǎo Becomes ní hǎo

The first phrase almost every learner meets is 你好 (nǐ hǎo) — “hello” — and almost nobody is told that it isn’t pronounced the way it’s written. In natural speech it comes out as ní hǎo, with the first syllable rising like a second tone. This is third-tone sandhi, the most important of Mandarin’s tone change rules, and understanding it clears up a whole family of “why does the recording sound different?” confusions at once.

The 3+3 rule

The rule is short enough to memorize in one breath:

When a third tone is immediately followed by another third tone, the first one is pronounced as a second tone: 3 + 3 → 2 + 3.

It is not optional, regional, or casual. It applies every single time, in careful speech and fast speech alike:

Written (underlying)	Spoken (surface)	Meaning
你好 (nǐ hǎo)	ní hǎo	”hello”
很好 (hěn hǎo)	hén hǎo	”very good”
水饺 (shuǐ jiǎo)	shuí jiǎo	”boiled dumplings”
可以 (kě yǐ)	ké yǐ	”may; can”
洗澡 (xǐ zǎo)	xí zǎo	”to bathe”

Notice the trap built into pinyin: it almost never shows this change. Dictionaries and textbooks write the underlying tones, so your eyes keep learning nǐ hǎo while every audio clip says ní hǎo.

Underlying tones vs. surface tones

The cleanest way to think about sandhi is to separate two layers. The underlying tone is what the syllable is — the tone stored in your memory and printed in the dictionary. 可以 is underlyingly third + third. The surface tone is what actually leaves your mouth once the rule has applied — here, second + third.

This matters because tones don’t change in the dictionary; they change in context. The syllable 好 (hǎo) is a third tone, but in 你好 the first syllable changes, not 好. Sandhi looks leftward: it’s the earlier third tone that becomes a rise, while the final third tone keeps its low, dipping shape. So you are not relearning words — you are learning one predictable adjustment that the language performs for you.

Why the change happens

The full third tone is the longest and most acrobatic of the four: it dives to the bottom of your pitch range, scrapes along, then climbs back up. (For why this tone is so hard to control, see /blog/why-mandarin-tones-are-hard/.) Doing that twice in a row is genuinely clumsy — your voice would have to dive, surface, and dive again. Mandarin’s solution is to turn the first dip into a simple rise, which is exactly a second tone. Sandhi isn’t an exception to fight; it’s the language smoothing out an awkward sequence.

Dumplings versus sleep

The classic cautionary pair shows why this is worth getting right:

• 水饺 (shuǐ jiǎo) — “boiled dumplings” — underlyingly 3 + 3, so it surfaces as shuí jiǎo (rise, then low dip)
• 睡觉 (shuì jiào) — “to sleep” — 4 + 4, two sharp falls

On paper they look unrelated. But if your sandhi’d third tone wanders or your fourth tone starts too low, “我想吃水饺 (wǒ xiǎng chī shuǐ jiǎo) — ‘I’d like to eat dumplings’” can drift toward wǒ xiǎng shuì jiào — “I want to sleep.” Drill the contours until the rise-then-dip of 水饺 feels nothing like the double fall of 睡觉.

How to hear the change

The fastest way to internalize sandhi is to make the unchanged version sound wrong to your own ear. Try this:

1. Listen for the rise. Play 你好 and 可以 a few times and track the first syllable — it lifts, exactly like a second tone, not a dip.
2. Compare a true 2+3 with a sandhi’d 3+3. 啤酒 (pí jiǔ) — “beer” is underlyingly second + third; 雨伞 (yǔ sǎn) — “umbrella” is third + third, so it surfaces as yú sǎn. Tonally they now rhyme — noticing that match is proof the rule has clicked.
3. Say the changed form out loud. Pronouncing the word the way it’s written (a full nǐ dip before hǎo) should start to feel as clumsy as it actually is.

Don’t compute it live

What about three third tones in a row, as in 我很好 (wǒ hěn hǎo) — “I’m fine”? Speakers don’t recalculate the rule syllable by syllable mid-sentence — they group words into chunks first, then apply sandhi inside each chunk. The reliable approach is to learn high-frequency 3+3 words as single, memorized melodies: ní hǎo, hén hǎo, ké yǐ. Longer chains, which deserve their own treatment, then mostly take care of themselves.

For practice tips and the half-third tone that travels alongside this rule, the companion post /blog/third-tone-sandhi/ goes deeper, and the half-third shape itself is covered in third tone pairs and the half-third.

Next we’ll tackle exactly those longer strings of third tones in multiple third tones and chunking, where grouping by meaning becomes the whole game.