I have read the following sentence in an Italian learning tool:
Io sono più vecchio di quanto tutti credano.
Is the subjunctive mood correctly used here? If so, why is it used?
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Sign up to join this communityDi quanto tutti credano is a proposizione comparativa. Normally (but there are plenty of exceptions in these topics) you use the indicative for the comparative of equality and the subjunctive for the comparative of majority or minority. From Serianni, Grammatica, XVI.227-233
[The comparative clauses of equality] have normally, in the explicit form, the indicative, although they allow also the conditional. [...] [The comparative clauses of inequality] allow the subjunctive, the indicative (today an indication of the least formal register, although once the indicative was used without restricitions) or the conditional if one wants "to emphasize the presence of the hypothesis in the action of the subordinate" (HERCZEG 1977: 343).
Some examples:
Oggi ho mangiato tanto quanto ho bevuto
Oggi ho mangiato più di quanto abbia bevuto
Oggi ho mangiato più di quanto ho bevuto (informal)