1.3. Simple Vowels
A simple vowel, i.e., a vowel that is not a diphthong, may be either long or short depending on its context and on whether or not there is a circumflex accent. There is a difference of duration and also of pronunciation between corresponding long and short vowels. Here are the rules for when vowels are short, and when they are long:

long vowels
Simple vowels (monophthongs) in stressed syllables take the long pronunciation when one and only one of the following consonants appears in the same syllable as the vowel (e.g., when word-final): b, d, g, (single) f, dd, some cases of l, n, m, and ng - and also when no consonant at all follows the vowel.

译文:
1.3.简单元音
一个单元音，不是指双元音，既可以是长音也可以是短音，视其发音情况，并就是否有音调口音，发音和发音持续的时间长短是有区别的。这里有规则，什么时候发短音，什么时候发长音：
长元音
简单的元音（单母音）在同一音节以一个辅音结尾的时候，强调音节时采取发长音，例如在单词结尾以b,d,g,单写的f,双写的dd,有些时候以l,n,m和ng结尾,甚至有时就根本没有辅音跟随的情况发长音。