Sundanese language
In 1873, Coolsma published a grammar of the Sundanese language, titled "Handleiding bij de beoefening der Soendaneesche taal" ("Manual for the Writing of the Sundanese Language"). He reluctantly used a transcription method developed by K.F. Holle and proscribed by the colonial government. That year he was tasked with translating the New Testament into Sundanese. He left his school in the hands of fellow missionary D. J. van der Linden and went to Sumedang; there he worked on his translation over a period of three years.