To avoid errors when referring to the clergy, it is necessary to have some knowledge. It is unlikely that the clergy will correct you if you refer to him with the words "Hello, father." However, an Orthodox person must know and follow the Church etiquette.
In Orthodoxy there are three degrees of the priesthood. The youngest is a deacon or assistant priest (deacon in monasticism). He does not have the powers of grace, what have the priests, so it is not suitable for the blessing. The correct address to the deacon, "Father deacon."
The next level of the priesthood is priests. The white clergy: priest (priest, pastor), Archpriest, Protopresbyter. In the black clergy, i.e., monks, is this: monk, Abbot, Archimandrite. When referring to the priest asking for the blessing: "Bless, father".
Turning to monk, the hegumen and Archimandrite may look like: "Bless, father" or "Bless, Reverend father". The latter is more correct, as in Orthodoxy it is not accepted to address with the words "Holy father", although in practice the treatment used by the laity quite often. If you know the name, please: "Bless me, father Nicholas." Of course, the name can be different. In a formal setting, and in writing, to the priesthood should be treated with the words: "Reverend", to the Abbot and Archimandrite – the "very Reverend".
The third level of the priesthood is the bishops (the bishops). Sana'a there are the following: Bishop, Archbishop, Metropolitan, Patriarch. All these Sana'a is only in the black clergy. The Bishop made contact with the words "Your Eminence". To the Archbishop or Metropolitan – "Your Eminence". To The Patriarch: "Your Holiness." If communication takes place in a more intimate setting, it is possible to use "master".