Passage: “Quam autem civitati carus fuerit, maerore funeris indicatum est.” Cic. Amic. 11
My translation in English: «Moreover, how dear he was to the citizenry was indicated by the grief of his funerals.»
My translation in French: “De plus, le chagrin éprouvé lors de ses funérailles indiqua à quel point il était cher au corps citoyen.”
Question: why is “fuerit” in the subjunctive mood? This does not look like an independent use of the subjunctive, nor like a future perfect, so can “quam” introduce subjunctive adverbial clauses ? To what syntactical category would these clauses belong?