Ave et Pax vobiscum!
I have a question about the Agnus Dei, which starts and ends thus: “Agnus Dei… miserere eis,” or as conventionally translated: “Lamb of God… have mercy on him (?), her (?), them (?).”
My question is, should it not be “… miserere eum (him), eam (her), eosdem (them)?”
Apologies to all who may have felt offended/ excluded/ prosletyzed by this post, but my intention is linguistic learning, even though the example happens to be religious!
The verb misereor usually takes a genitive object in classical Latin, but it can also take a dative or even accusative object.
Isn’t it “miserere nobis”? Anyway, it’s a passive form (deponent). Not really something we have in English, but imagine saying “be miseried for us”.
Gratia vobis H and JE! Genitive it is.
Also, you’re right JE, at most times (eg. Sunday Mass), it would end with “… miserere nobis (… mercy on us).”
But if it were a Requiem Mass (Missa pro Defunctis) it would be “… mercy on him/ her.” Also, if one were saying a private prayer for a departed person or persons, the object would be 3d person singular or plural as the case may be.
So, if I’m understanding you fellas right, the correct options would be:
Case 1, if reciting a prayer for Grandpa (3d person genitive masculine): “… miserere eis.”
Case 2, if for Grandma (3d p gen fem): “… miserere ea.”
Case 3, if for Grandpa and Grandma (3d p genitive plural): “…miserere eum.”
Pls correct me if I’m wrong, and thanks y’all again!
I’m just a Latin beginner, but I don’t believe that’s right. If you’re trying to follow the “miserere nobis” of the Mass, I would think that you would want to use dative. ei is the dative singular form for masculine and feminine. For plural it’s eis for both genders. So singular “miserere ei” or plural “miserere eis”.
If you’re trying to copy the “miserere mei” of Psalm 50/51, you’ll want to use the genitive. Singular eius. Plural eorum (masculine), earum (feminine). So “miserere eius” or “miserere eorum/earum”.
Thanks again JE! I just realized H said classical Latin uses the genitive in this case, but since Church Latin is post-classical, then the dative you recommend is the way to go.
Grammar aside, what is sung at a Requiem Mass is
Agnus Deī, quī tollis peccāta mundī, dōnā eīs requiem.
Agnus Deī, quī tollis peccāta mundī, dōnā eīs requiem.
Agnus Deī, quī tollis peccāta mundī, dōnā eīs requiem sempiternam.
See Allen and Greenough, Latin Grammar, 146 for the paradigm of is, ea, id.
Gratia tibi B!
Welcome. If I may dare a few corrections, first, since the expression is grātiās alicui agere, I believe you could drop the verb, but you should retain the accusative plural: grātiās tibi. Second, it’s odd to have both avē (which is used for a single person) and pāx vōbīs (which is used for a group of two or more) in the same salutation. You could move everything to the plural, if you like it: avēte et pāx vōbīs. Finally, the correct Latin is in hāc lacrimārum valle.
Gratias tibi B! (see, I listen!) 
I amended my location (which originally said: “In this tearful goodbye” instead of “In this tearful valley,”), and will conform future salutations as you recommend.
Corrections and critiques are much appreciated!
Welcome. Still missing:
haec → hāc
Oh haec this is difficult, but if I’m going to learn I’ll need to hac it!
Gratias tibi!