Why "tu" form, ie Jazzy's answer, in Sp greeting buenas tardes etc other latin langua

This has nothing to do with the "tu" form. These conventional greetings are in the plural because the Spanish language has remained faithful to the Latin original. Most Spanish words came not from the nominative case in Latin, but from the accusative case (i.e. object, as opposed to subject) ? Buenos ? in ?buenos d铆as? in fact comes from ?bonos?, the accusative plural. The word ?dies/-ei? in classical Latin, was a fifth declension word and the accusative plural was ?dias?. ?Buenos d铆as? comes from the Vulgar Latin form ?bonos dias? and the fact that it remained plural rather than singular appears to be a quirk of language. A similar quirk is evident when you compare the Spanish and Portuguese ways of asking the time, as in Spanish (like English, you ask ??qu茅 hora es?? in the singular and in Portuguese ?que horas s?o?, in the plural.