In chapter XXII of Lingua latina per se illustrata: Colloquia Personarum, I have read the following sentence (emphasis mine in the word I find difficult to understand):
Hic anulus ex auro puro factus est, anulus tuus ex ferro, quod tenui auro operitur.
I don't understand why the relative pronoun quod is in nominative or accusative case. I think it should be in ablative case, since it refers to ferro, so it should be quo. Am I right? Or am I missing something?
A relative pronoun agrees with its antecedent in gender and number. Its case is determined by its role in the relative clause.
In this case, quod agrees with ferrum in gender (neuter) and number (singular). In the relative clause, though, it functions as the subject, so it's in the nominative case. Here's a possible translation:
This ring was made from pure gold; your ring [was made] from iron, which is covered with thin gold [i.e. a thin layer of gold].
To understand why quod isn't ablative, you could try recasting the relative clause, substituting ferrum for the relative pronoun:
ferrum tenui auro operitur.
The iron is covered with thin gold.
