A predicate substantive agrees with its subject in case: Μιλτιάδης ἦν στρατηγός
A predicate substantive may agree in gender and number with its subject; but this is often impossible: τύχη τὰ θνητῶν πρά_γματα
πάντ' ἦν Ἀλέξανδρος
A predicate substantive or adjective agrees with the subject of the governing verb when the subject of the infinitive is omitted because it is the same as that of the governing verb ( cross937):
οὐχ ὁμολογήσω ἄκλητος ἥκειν
εἴπερ ἀξιοῦμεν ἐλεύθεροι εἶναι
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].