In Chinese Pinyin System, the function of y and w is to provide sound insulation. These letters are used when a syllable begins with a zero initial (no initial consonant).
When the syllable is in zero initial and there is another vowel after the vowel i, change i to y.
☞ ia → ya | ie → ye | iao → yao | iou → you
☞ ian → yan | iang → yang | iong → yong
If there isn't another vowel after vowel i, add y before i.
☞ i → yi | in → yin | ing → ying
When the syllable is in zero initial and there is another vowel after the u, change u to w.
☞ ua → wa | uo → wo | uai → wai | uei → wei
☞ uan → wan | uen → wen | uang → wang | ueng → weng
If there isn't another vowel after vowel u, add w before u.
☞ u → wu
When the syllable is in zero initial, whether there is another vowel after the ü or not, add y before ü, and the two points (¨) on the ü must be omitted.
☞ ü → yu | üe → yue | üan → yuan | ün → yun
Summary: The letters y and w serve as insulation letters to ensure that syllables beginning with vowels are clearly distinguished. They are added or modified when a syllable has no initial consonant, preventing confusion in pronunciation and spelling.