Question about Latin vocative

[Daniel]

Psalm 21 (Challoner translation) begins, "O God my God, look upon me: why hast thou forsaken me? Far from my salvation are the words of my sins. O my God, I shall cry by day, and thou wilt not hear: and by night, and it shall not be reputed as folly in me."

In the Vulgate, this is, "Deus, Deus meus, respice in me : quare me dereliquisti? Longe a salute mea verba delictorum meorum. Deus meus, clamabo per diem, et non exaudies; et nocte, et non ad insipientiam mihi."

But I see on Wiktionary's meus page that the vocative case of "meus" is "mi". And doesn't the case of the adjective need to agree with the case of the noun? So I'm wondering, why is it "Deus meus" rather than "Mi Deus" / "Deus mi"?

One other question: I see that there's a vocative particle "o" in Latin (same as English "O" apparently). But, when exactly do you need to use it? Is it always optional?

[Jayne]

The usual form of the vocative of meus is mi as you say.  However, there are rare occurences of meus used as a vocative.  These tend to be in poetic settings rather than prose.

One other question: I see that there's a vocative particle "o" in Latin (same as English "O" apparently). But, when exactly do you need to use it? Is it always optional?

In theory, vocative is shown by a vocative case ending. However, other than second declension masculine, the vocative is identical to the nominative.  This means that in most instances, one uses the context to determine vocative   So, if an author wanted to be especially clear that he was using vocative, it would be useful to use the particle "o".  But it would not be grammatically wrong to leave it out, although Classical authors tended to avoid using "o" before words that start with a vowel.

Sometimes "o" indicates emphasis or emotion.  It can also be used when a long clause has a vocative function, for example the Latin equivalent of "o you who read traditional Catholic forums while drinking coffee."

[ludimagister]

Apart from anything else, Deus meus has more of a beseeching tone than Deus mi.

[Jayne]

Apart from anything else, Deus meus has more of a beseeching tone than Deus mi.

This is exactly what is going on.  It is a stylistic choice based on how it sounds.  Pretty much every time a Latin author uses meus as vocative this is the sort of reasoning involved.

[Daniel]

Ok, thanks.

New question (this time not about the vocative): I know that "meum" is an adjective that means "my"/"mine". But I'm also aware that the singular first person pronoun has a genitive form "mei" which (I presume) also means "my". So, if I wanted to say e.g. "my cat" (in the nominative), would I use "cattus meus" or "cattus mei"? What exactly is the difference? (Pretty sure my past Latin teacher explained this at one point, but I've since forgotten )

Also, why does "mei" occur in the phrase "miserere mei" ("have mercy on me")? I could see it needing the dative or maybe the accusative, but I'm not understanding why the genitive would be used. Unless it's some sort of idiom or other anomaly?
edit - I've also noticed sort of the opposite version of this, in the phrase "Crux sacra sit mihi lux / non draco sit mihi dux." From what I can tell, it should be translated as "may the holy Cross be my light / mayn't the dragon be my leader", however, if that's that then why is the dative "mihi lux"/"mihi dux" being used, instead of the genitive "mei lux"/"mei dux" or the adjective "mea lux"/"meum dux"?

[Christina_S]

Ok, thanks.

New question (this time not about the vocative): I know that "meum" is an adjective that means "my"/"mine". But I'm also aware that the singular first person pronoun has a genitive form "mei" which (I presume) also means "my". So, if I wanted to say e.g. "my cat" (in the nominative), would I use "cattus meus" or "cattus mei"? What exactly is the difference? (Pretty sure my past Latin teacher explained this at one point, but I've since forgotten ) "Mei" is literally "of me," and as such it isn't frequently used. If you are describing the image of yourself, you would probably say "imago mei," but anything external, including physical features, words, and possessions would use "meus, -a, -um." So "cattus meus" would be fine, but "feles" is the Latin word for cat. "Feles mea" 

Also, why does "mei" occur in the phrase "miserere mei" ("have mercy on me")? I could see it needing the dative or maybe the accusative, but I'm not understanding why the genitive would be used. Unless it's some sort of idiom or other anomaly?
Wheelock's is telling me it might be a predicate genitive, which means that a genitive following a verb is describing the subject in some way. I may be wrong though.
edit - I've also noticed sort of the opposite version of this, in the phrase "Crux sacra sit mihi lux / non draco sit mihi dux." From what I can tell, it should be translated as "may the holy Cross be my light / mayn't the dragon be my leader", however, if that's that then why is the dative "mihi lux"/"mihi dux" being used, instead of the genitive "mei lux"/"mei dux" or the adjective "mea lux"/"meum dux"?
You might think of it this way:  "May the holy Cross be light to/for me." This is a typical Latin construction, too. When introducing oneself in Latin, proper construction is, "Nomen mihi ... est," rather than the English possessive, "MY name is..." We tend to slack when translating datives into modern English, but the common translation is easier to remember, I guess.

[Jayne]

Ok, thanks.

New question (this time not about the vocative): I know that "meum" is an adjective that means "my"/"mine". But I'm also aware that the singular first person pronoun has a genitive form "mei" which (I presume) also means "my". So, if I wanted to say e.g. "my cat" (in the nominative), would I use "cattus meus" or "cattus mei"? What exactly is the difference? (Pretty sure my past Latin teacher explained this at one point, but I've since forgotten )

You would say "cattus meus".  The pronoun mei is virtually never used to mean "my".  It is used in contexts in which one would say "of me" such as "Part of me wants to hide in a cave and part of me wants to scream."  This is a partitive genitive construction and is pretty rare in the singular. ( The construction is common when using first person plural in expressions like "some of us," "many of us," none of us," etc.  )There are also some verbs that take genitive objects, such as verbs of remembering and forgetting.  One would use mei with these.

Also, why does "mei" occur in the phrase "miserere mei" ("have mercy on me")? I could see it needing the dative or maybe the accusative, but I'm not understanding why the genitive would be used. Unless it's some sort of idiom or other anomaly?

This is one of the aforementioned verbs that takes a genitive object.  There is a group of verbs used in an impersonal construction to discuss feelings.  Some others are pigere, pudere, paenitere, taedere. The object of the feeling is expressed with the genitive.

edit - I've also noticed sort of the opposite version of this, in the phrase "Crux sacra sit mihi lux / non draco sit mihi dux." From what I can tell, it should be translated as "may the holy Cross be my light / mayn't the dragon be my leader", however, if that's that then why is the dative "mihi lux"/"mihi dux" being used, instead of the genitive "mei lux"/"mei dux" or the adjective "mea lux"/"meum dux"?

This seems to be a dative of possession, a fairly common construction formed with esse.  Using the genitive emphasizes the possessor; the dative, the fact of possession.

I see there has been another answer posted while I was writing this but I might as well post this since it is already written.

[ludimagister]

Also mei (or any genitive personal pronoun) can be used in an objective sense:

Tuus amor mei: your love for me.

In fact, any genitive may be used in this way. Amor patris is ambiguous: it may mean "my father's love (for me)" or "(my) love for my father". To avoid the ambiguity an adjective can be used in the subjective sense: amor paternus: my father's love. But this is mainly poetic.

[Jayne]

So "cattus meus" would be fine, but "feles" is the Latin word for cat. "Feles mea" 

It is true that feles is a Latin word for cat, probably the most common one in the classical period.  Both catus and cattus occur too, moreso in other periods. The scientific name for the domestic cat is felis catus using both.

[Daniel]

So "cattus meus" would be fine, but "feles" is the Latin word for cat. "Feles mea" 

It is true that feles is a Latin word for cat, probably the most common one in the classical period.  Both catus and cattus occur too, moreso in other periods. The scientific name for the domestic cat is felis catus using both.

Felis catus, a.k.a. Felix the Cat. If I remember correctly, there's also a Felis silvestris catus which has Sylvester thrown into the mix (not sure what silvestris means unless it's related to silva...).
I actually had known "feles", but I didn't use it in my example because I couldn't remember whether it was masculine or feminine and also couldn't remember whether it was spelled "feles" or "felis". But "cattus" I knew off the top of my head, because it's like the "cat". (I'll now have to add "catus" to my vocabulary, lol)

[aquinas138]

So "cattus meus" would be fine, but "feles" is the Latin word for cat. "Feles mea" 

It is true that feles is a Latin word for cat, probably the most common one in the classical period.  Both catus and cattus occur too, moreso in other periods. The scientific name for the domestic cat is felis catus using both.

Cattus is certainly used in the modern period 

[Christina_S]

Ah, I see. Thank you for the correction.