The following site provided me with the best explanation I've found so far for using dernier and prochain before or after the noun:
https://languagecenter.cla.umn.edu/lc.... (Unfortunately, the site for me doesn't display letters with accents on them, so it's a bit difficult to read.) However, in summary, they say: AFTER the noun = the most recent or the next one coming up (usually a unit of time). BEFORE the noun = the latest in a SERIES or the next in a SERIES. So, for example, when dernier is used AFTER the noun, in English we would say LAST YEAR (no article). When dernier is used BEFORE the noun, in English we would say THE LAST YEAR (with an article). Some example sentences for AFTER NOUN use: (1) L'année dernière, mes parents m'ont donné une nouvelle voiture. Translation: Last week, my parents gave me a new car. (2) La semaine prochaine, mes parents vont m'acheter une nouvelle voiture. Translation: Next week, my parents will buy me a new car. Example sentences for BEFORE NOUN use: (1) La dernière année de sa vie était sa plus heureuse. Translation: The last year of his life was his happiest. [In this case “la dernière année” is not “last year” (the year before this one), but “the last year of his life” (i.e. the last of a series, the last year in the series of years that make up his life)]. (2) Le prochain examen sera moins difficile. Translation: The next exam will be less difficult. [The series is implied: all exams to be taken this semester.] Other example SERIES phrases BEFORE NOUN: The last cake I ate. The last wife of Henry VIII. My last football game. The last book I read. . . . The explanations on this site at least fit the usage in Fluenz. "Son bureau est au dernier étage de ce bâtiment." (His office is at the last floor of this building - THE last floor of a SERIES of floors - so dernier came BEFORE the noun.) Anyway, hope some of the above is useful.