SPN III Chapter 1 Lesson 3 (Irregular Nouns - TB p. 43)

Rule 1: Nouns that end in "ma" are derived from Greek roots, and are considered masculine nouns and use masculine articles (el/los, un/unos). Other similar nouns that are considered masculine are: el día, el mapa, el planeta.
Rule 2: Irregular form: la mano
Rule 3: la fotografía ---> la foto