So you thought that Latin was gonna be easy for you: Amo, Amas, Amat, et cetera!

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Father Z, [WHO ELSE] provides a venue for Latin usage today and one of his acolytes provides the answers: How do I know when to end a prayer with the short form "Per Christum Dominum nostrum" and when to use the long form "Per Dominum Nostrum Jesum Christum, Filium Tuum, qui tecum vivit et regnat in unitate Spiritus Sancti, Deus, per omnia saecula saeculorum"?

I. If the prayer is addressed to God the Father:

Per Dominum nostrum Jesum Christum, Filium tuum: Qui tecum vivit et regnat in unitate Spiritus Sancti Deus: per omnia saecula saeculorum. R. Amen.

II. If the prayer is addressed to God the Father, but God the Son is mentioned at the beginning of the prayer:

Per eundem Dominum nostrum Jesum Christum, Filium tuum: Qui tecum vivit et regnat in unitate Spiritus Sancti Deus: per omnia saecula saeculorum. R. Amen.

III. If God the Son is mentioned at the end of the prayer:

Qui tecum vivit et regnat in unitate Spiritus Sancti Deus: per omnia saecula saeculorum. R. Amen.

IV. If the prayer is addressed to God the Son:

Qui vivis et regnas cum Deo Patre in unitate Spiritus Sancti Deus: per omnia saecula saeculorum. R. Amen.

V. If God the Holy Ghost [that includes "God the Holy Spirit", too] is mentioned in the prayer:

Per Dominum nostrum Jesum Christum, Filium tuum: Qui tecum vivit et regnat in unitate eiusdem Spiritus Sancti Deus: per omnia saecula saeculorum. R. Amen. W.D.T.P.R.S.