I believe your native Cantonese has both sounds, so you’re probably just asking about the spelling rule in Hanyu pinyin. If the ü sound (IPA /y/) is after L or N, then it is written with the two dots (umlaut): lü, nü. If you don’t have the ü symbol, you could theoretically use the letter V instead (lv, nv). However, if the ü sound comes after a palatal consonant (y, x, q, j), then the umlaut is unwritten, since palatal consonants can only be followed by /y/ or /i/, never /u/. So, yu, xu, qu, and ju are all pronounced with the ü sound, even thought the two dots are not written.

包含“'ü'” 的字有 遇、雨、举、剧、女、旅、绿、去、曲、须、旭等等。 但在j, q, x, y后面，ü的两点是不写的,，写成u，这是韵母中的一项规则。 包含'“u'” 的字就更多了，有 路、卤、努、母、牧、图、兔、睹、读、住、煮、厨、初、书、属、步、补、普、瀑、粗、醋、福、复、酷、哭、如、入、苏、速、五、武、组、足、固、古等等。

thank you much clear

when the "ü" sound is after“j"/"q"/"x"/"y", you should get rid of two dots, like this: "ju"/"qu"/"xu"/"yu". but if the "ü" sound is after “l”/"n", the "ü" should be written as "v" and the sound still is "ü",like this: "lv"/"nv".

好的 謝謝