Revelation 21:3a

καὶ ἤκουσα φωνῆς μεγάλης ἐκ τοῦ θρόνου λεγούσης

And I heard a loud voice from the throne saying....


 * why does the verb ἤκουσα take a genitive φωνῆς μεγάλης as a direct object?
 * And because of this, the participle λεγούσης also becomes genitive to agree.
 * Same as in Revelation 11:12 καὶ ἤκουσαν φωνῆς μεγάλης ἐκ τοῦ οὐρανοῦ λεγούσης αὐτοῖς
 * But then there is a different pattern Revelation 12:10 καὶ ἤκουσα φωνὴν μεγάλην ἐν τῷ οὐρανῷ λέγουσαν where ἤκουσα now takes accusative φωνὴν μεγάλην and the participle becomes accusative λέγουσαν to agree.
 * Apparently this is a type of "genitive of direct object" and this is seen with certain verbs, especially verbs of the five senses.
 * According to another grammar, there are fine nuances. To see, feel or hear a whole object uses an accusative. To see, feel or hear a part, or to treat the object as a source, the genitive is used. But the grammar notes that these nuances were lost colloquial hellenistic greek, and just retained as a habit of language.
 * These nuances are not very apparent in the above verses. In all cases, the voice is a source of the message. But sometimes genitive and sometimes accusative.