Mandarin tone sandhi: the rules learners actually need

What sandhi is

Tone sandhi is the rule by which a syllable's tone changes depending on what tone follows it. The change happens automatically in fluent speech; native speakers don't think about it, and Pinyin (in its citation form) doesn't write it. Learners typically hit sandhi when they say something correctly tone-by-tone and a native listener still asks them to repeat it — the contour was right on paper but not the contour spoken Mandarin actually uses.

There are dozens of fine-grained sandhi phenomena in Mandarin, but three rules account for almost everything a HSK 1–3 learner needs. We'll cover those, and skip the corners until you're a level or two further in.

Rule 1: 3-3 → 2-3 (the big one)

When a third-tone syllable is followed immediately by another third-tone syllable, the first one is pronounced as a second tone. The second one stays as a third. Citation form and spoken form diverge:

This is the most common sandhi a beginner hits, because some of the highest-frequency words in Mandarin are 3-3 pairs: 你好 (hello), 很好 (very good), 可以 (can / OK), 老虎 (tiger). Tone-pair drills in this app explicitly include the 3-3 group so you can train your ear on it directly.

The rule chains. In a 3-3-3 sequence, the first two syllables both go to 2: 我很好 (wǒ hěn hǎo, "I'm fine") is spoken closer to wó hén hǎo. Don't memorise the chain forms — drill the pair-level rule until it's automatic, and the chain takes care of itself.

Rule 2: 不 (bù) before fourth tone

不 is lexically fourth tone. When the next syllable is also fourth tone, 不 sandhies to second tone. Before any other tone (or in isolation), 不 stays as fourth.

The pattern surfaces constantly: 不是 (bú shì, "is not"), 不会 (bú huì, "won't / can't"), 不要 (bú yào, "don't want"). Before non-fourth tones — 不大 (bù dà, "not big"), 不多 (bù duō, "not much") — 不 keeps its fourth tone.

A trap: many textbooks and dictionaries cite the lexical fourth tone of 不 even in phrases where every speaker says it in second tone. The spoken-form line in the demo above is what you'll actually hear.

Rule 3: 一 (yī) — the tone changer

一 is lexically first tone, but in connected speech it changes by the same logic as 不:

• Before fourth tone, 一 becomes second.
• Before first, second, or third tone, 一 becomes fourth.
• Alone (or as an ordinal number), 一 stays as first.

一 is everywhere — counting, time, measure phrases, common compounds — so internalising its sandhi early pays back quickly. The rule is mechanical: look at the next syllable's tone, apply.

How to drill it

Sandhi is an ear skill, not a vocabulary skill. Three suggestions:

• Use the Tone Pair drill and pay attention to 3-3 pairs specifically. The drill plays connected audio, so what you hear is the sandhi-applied form, and the answer is the citation-form pair. Don't trust your eyes — listen for the contour.
• When you encounter a phrase with 不 or 一 in vocabulary review, say the citation form once, then say it again at speaking speed. Your second pass should drift to the sandhi-applied form automatically; if it doesn't, repeat until it does.
• Don't try to write sandhi-applied pinyin in your notes. Pinyin is a citation-form system. Writing ní hǎo in your flashcards will confuse you later when you see nǐ hǎo in any textbook or dictionary. Drill the spoken form, write the lexical form.

What we left out

Mandarin has further sandhi rules — half-third tone in pre-1st/2nd/4th positions, the tone changes around 啊 (a), specific corner cases for reduplicated verbs and adjectives, and the lexical-vs-grammatical tone of 着 / 了 / 过 in unstressed positions. None of these are essential at HSK 1–3, and chasing them too early just makes the rule list feel arbitrary. Get fluent with the three rules above first; revisit the corners when you start hearing them in native input.