مَا الْمُعَلِّمُ غَائِبًا
Translation: The teacher is not absent.
مَا: working negative particle.
الْمُعَلِّمُ: definite, nominative ism mā.
غَائِبًا: accusative khabar mā.
This is an added teaching example, not a quotation from the book. It illustrates why the ism of working mā may be definite.