The "ü" vowel
In series Pinyin quick start guide
So you've learned 5 vowel sounds already, but those are not all the vowel sounds in Mandarin Chinese! The “ü” vowel is a totally different sound, and it does not exist in English. If you speak French or German you might be familiar with this sound, but otherwise, you probably have to train your mouth to make a whole new sound.
Pinyin's "ü" Vowel Sound
To make pinyin's "ü" sound, make the pinyin "yi" sound (or the English "ee" sound), and then slowly round your lips. That's all there is to it! Your tongue needs to stay tense like it is when you make the "yi" sound, but your lips must be rounded. If you're having trouble making the sound, it's probably because you let your tongue relax. (The tongue is totally relaxed when you make an English "oo" sound, but you need it to stay tense, like it is for the "ee" sound.)
|yu||yu is just the pure "ü" sound by itself. The "y" is added to make the syllable boundary clear, and the two dots are not written for yu. The other sounds in the same column, nü and lü, rhyme with yu.|
|yue||yue is simply "ü" + "eh" (similar to ye). Don't write the two dots on "ü" for yue. The other sounds in the same column, nüe and lüe, rhyme with yue.|
|yuan||yuan is "ü" + "en" (similar to yan). Don't write the two dots on "ü" for yuan.|
|yun||yun is simply "ü" + -n, but it might sound better to pronounce it as yu + yin. Don't write the two dots on "ü" for yun.|
When to Write the Dots
The answer is just for convenience (in other words, pure laziness). The two dots are added only when an initial can combine with both -u and -ü. So there is a nu and a nü, a lu and a lü. Only for those two pairs, if you didn't add the two dots when you meant "ü", there would be total confusion. Then nüe and lüe also get the dots for consistency with nü and lü, even though there are no "nue" and "lue" to confuse nüe and lüe with.
Pinyin Chart Fragment
This is just a part of the full pinyin chart, limited to the sounds we've just covered.
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Notice that the chart above is mostly empty. That's because pinyin's "ü" sound really likes to combine with the initials "j-", "q-", and "x-", which you'll learn in the next and final section. For this section, be sure to really learn the "ü" sound in isolation, which will usually be written as "yu" in pinyin.
Just one more part left! The "j" "q" and "x" sounds is the final section.