First, the de/di phenomenon is not 文白异读 (literary-colloquial
distinct readings). The term "文白异读 (literary-colloquial distinct
readings)" have strict academic definition. Not every homograph
is 文白异读 (literary-colloquial distinct readings). Not every
literary-colloquial distinction is 文白异读 (literary-colloquial
distinct readings). Books on Chinese phonology do not list de/di
as 文白异读 (literary-colloquial distinct readings). The differences
between 文白异读 (literary-colloquial distinct readings) are
systematic in nature. For instance, there is an (u)o/ao
relationship: 薄 bo bao 剥 bo bao 落 luo lao 烙 luo lao 凿 zuo zao
Such a systematic relationship is not a coincidence. This is so
because 文白异读 (literary-colloquial distinct readings) reflects
the contact of two sound systems. It is systematic in nature.
But it is not the case with de/di. There is no systematic
relationship with the e/i pair. So what is it? "De" is just a
weak form of "di". In Chinese, words seldom have weak form. But
"的" is so common (it is the most comm