Core rule: In the basic construction, a diptote takes nominative ḍamma and accusative/genitive fatḥa.
Book example: «جَاءَنِي عُمَرُ، وَرَأَيْتُ عُمَرَ، وَمَرَرْتُ بِعُمَرَ».
Important caution: «عُمَرَ» can be either accusative or genitive depending on the governor. No ordinary tanwīn occurs. With «ال» or as a muḍāf, a diptote can show genitive kasra.
The next lesson will treat the author’s fourth case class, in which certain nouns use letter-based signs.