"Koudang" used to be a simplified word for "dang", but it was no longer used. The standard word should be "dang" (its standard word code is 04 18, and the traditional Chinese characters are "dang" or "dang"). When the meaning is "when", read "dā ng"; When the meaning is "dang", there are three pronunciations of "dāng dàng dang".
The pronunciations and meanings of "dang" and "dang" are as shown in the following figure (from The Great Chinese Dictionary and The Sea of Chinese Words):
The Pronunciation and Meaning of "Dang"
Pronunciation of "dang"