sponsa furerat

In Exercitia Latina XLI, exercitium 1 Orberg has (Q.6) the following:

Ante adventum Aeneae Lavinia Turno, regi Rutulorum, sponsa fuerat.

I take it this is a purely adjectival use of the word rather than a participle but that ‘sponsa erat’ would have been just as good, right?

This is an alternative periphrastic form of the pluperfect.

In the print version of Allen & Greenough, this is explained in a footnote attached to the passive paradigm of amo (page 94). Unfortunately, the on-line version doesn’t seem to include the passive paradigm for this verb.

Thanks Quimmik