In the Pinyin system for romanizing Chinese, the vowel combinations 'iou', 'uei', and 'uen' are simplified to 'iu', 'ui', and 'un' respectively when they follow an initial consonant.
This simplification is applied because the medial vowels 'o' and 'e' in 'iou', 'uei', and 'uen' tend to become less prominent when preceded by a consonant. Omitting these medial vowels better reflects the actual pronunciation.
Examples:
☞ x－iōu → xiū (羞, shy; bashful)
☞ g－uèi → guì (贵, expensive)
☞ c－uēn → cūn (村, village)
When the final 'ü' or any finals in the 'ü' column are combined with the initials 'n' or 'l', the umlaut (two dots) over the 'ü' should NOT be omitted.
Examples:
☞ nǚ (女, female) | lǜ (绿, green)
☞ nüè (虐, cruel) | lüè (略, omit)
Conversely, when the final 'ü' or any finals in the 'ü' column are combined with other initials (excluding 'n' and 'l'), the umlaut over the 'ü' MUST be omitted.
Examples:
☞ j－ǚ → jǔ (举, to hold up)
☞ j－üé → jué (倔, stubborn)
☞ q－üán → quán (拳, fist)
☞ q－ǘn → qún (群, group)
These rules ensure that Pinyin remains both concise and true to the phonetic reality of Mandarin Chinese. By understanding when to omit medial vowels and the umlaut, you can write and pronounce Chinese Pinyin more accurately.