The Ü-Sound

You place an umlaut (¨) over the letter "u" when it comes after an initial "l" or "n" in case it should be necessary to represent the sound [y]. This need happens when you have to distinguish the front high rounded vowel in "lü" from the back high rounded vowel in “lu". Tonal markers are added on top of the umlaut, as in “lǘ".

However, the "ü" is not used in other contexts where it could represent a front high rounded vowel, namely after the letters "j”, “q”, “x” and “y”.

The Apostrophe

The apostrophe (') is used before a syllable starting with a vowel (“a”, “o”, or “e") in a multiple-syllable word when the syllable is not the beginning of the word. For example, 西安 is written as "xī’ān", while 先 is written as [xiān]. In very seldom occasions, the apostrophe is not used. For example, when the syllable comes immediately after a hyphen, for example, 第二 is written “dì-èr”, without an apostrophe.

The Use of “y” and “w”

"Y" and "w" are equivalent to the semivowel-medials "i”, “u” and “ü”. They are spelled differently when there is no initial consonant to mark a new syllable: "fanguan" is “fan-guan", whereas "fangwan" is “fang-wan". In addition, "y" and "w" are added to fully vocalic "i”, “u” and “ü" when these appear without any initial consonant so that they are written "yi”, “wu” and “yu”.