Chinese · Pronunciation Guide
lyo, said as one quick beat with a sharp falling pitch. Pinyin writes it liu, but the vowel in the middle is real and needs to be heard.
liù
A lone fourth tone with no sandhi. What it does trigger, though, is a change in others: because 六 is falling, a preceding 一 becomes rising, and 不 before it would also switch to a second tone. So the number is more often a cause of sandhi than a subject of it.
Listen: 六
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我六点回来。
Wǒ liù diǎn huílái.
I'll be back at six.
Listen: 我六点回来。
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我六点回来。 gives the low wǒ, then liù falling, then diǎn dipping — a fall immediately followed by a dip, which are easy to confuse if the fourth tone is not decisive enough. Wǒ stays low here because liù is not a third tone. The sentence closes on huílái, two rising tones in a row, lifting the whole thing at the end.
The frequent English rendering is "lee-oo" with two syllables, or "loo" with the middle collapsed away. Both miss it: the syllable is one beat containing a moving vowel. Learners also tend to use a dark, velarised l — the sound at the end of "full" — where Mandarin wants a bright forward one, which noticeably thickens the whole word.