When a syllable has compound finals (ao, iao, iou, uai, etc.), only one vowel receives the tone mark. The priority order is always a → o → e → i → u → ü.

If a syllable contains a and o, the tone mark sits on a. If a is missing but e is present, e carries the mark. Only when none of a/o/e exist does the mark drop onto i/u/ü.

Tip: Write the raw vowel stack (iao, iou, uei, etc.) before the tone is applied. Mark the priority vowel, then collapse the spelling (iao → iao, iou → iu, uei → ui) while keeping the tone on the same vowel.

Practice: Check mixed drills for words like 请坐 (qǐng zuò) and 朋友 (péngyou) to reinforce how the tone marks shift in multi-vowel syllables.

When a syllable starts with i/ü, Pinyin adds y (ya, yo, yue, yun). When it starts with u, Pinyin adds w (wa, wo, wei).

These letters are spelling helpers; they do not add extra sound.

Use an apostrophe when the next syllable begins with a/e/o or er to avoid ambiguity (Xī'ān, tiān'é, nǚ'ér).

This keeps syllable boundaries clear in writing and helps pronunciation.