We call the verbs that end with MASU "the MASU-form of verbs." We use the MASU-form, when we speak politely. To make it a negative, we change MASU to MASEN. So, YOMIMASU (to read) becomes YOMIMASEN.

The publication of textbooks for NHK’s radio lessons started in 1925, the same year radio broadcasting commenced. Taking the opportunity of the centennial of Japanese broadcasting, the NHK ...

You use DESHÔ at the end of a sentence, when you talk about a prediction for the future or something uncertain. When you use it in combination with nouns or adjectives, you replace DESU at the end of ...