The Holy Eucharist is the Sacrament which contains the body and blood, soul and divinity, of our Lord Jesus Christ under the appearances of bread and wine.
What do you mean by the appearances of bread and wine?
By the appearances of bread and wine I mean the figure, the color, the taste, and whatever appears to the senses.
When did Christ give His priests the power to change bread and wine into His body and blood?
Christ gave His priests the power to change bread and wine into His body and blood when He said to the Apostles, Do this for a commemoration of Me.
How do the priests exercise this power of changing bread and wine into the body and blood of Christ?
The priests exercise this power of changing bread and wine into the body and blood of Christ through the words of consecration in the Mass, which are the words of Christ: This is My body; this is My blood.