Special Spelling Rules of [i] / [u] / [ü]
As we already know, the vowels in the Pinyin system can be an independent syllable itself even without the Pinyin initials. But, there is an exception about the vowels [i], [u], and [ü].
When [i], [u] or [ü] makes a syllable by itself:
1. “y” should be added in front of [i] and [ü].
Note:
Note that the two dots above [ü] should be omitted as [u] in written form, but the pronunciation keeps as the original [ü].
e.g.
| Spoken pronunciation | Written form | 
| i | Yi | 
| ü | yu | 
2. “w” should be added in front of [u].
e.g.
| Spoken pronunciation | Written form | 
| u | wu | 
When [i] / [u] / [ü] starts a syllable without other initials:
1. When [i] starts a syllable, it is written as “y”.
e.g.
| Spoken pronunciation | Written form | 
| ia | ya | 
| ian | yan | 
| iang | yang | 
| iao | yao | 
| iou | you | 
| iong | yong | 
| ie | ye | 
Note:
Note that except in “in” and “ing”, “y” is added at the beginning.
| Spoken pronunciation | Written form | 
| in | yin | 
| ing | ying | 
2. When [u] starts a syllable, it is written as “w”.
e.g.
| Spoken pronunciation | Written form | 
| ua | wa | 
| uai | wai | 
| uei | wei | 
| uan | wan | 
| uang | wang | 
| uen | wen | 
| uo | wo | 
3. When [ü] starts a syllable, “y” should be added in front of it, and the two dots above [ü] are omitted which is written as [u].
e.g.
| Spoken pronunciation | Written form | 
| üe | yue | 
| ün | yun | 
| üan | yuan | 
