Who do I match numerically when using the possessive dative?

Question

When using the dative of a noun in combination with a form of the verb esse to indicate possession, should the verb be in the same number as the subject or as the predicate noun(s) when there is a disagreement between the two?

For example, which are correct?

Matching the subject numerically
Puellis sunt rosa. (The girls have a rose.)

Discipulo est multa pocula. (The pupil has many cups.)

...or...

Matching the predicate numerically
Puellae sunt rosae. (The girl has roses.)

Discipulis est poculum parvum. (The pupils have a small cup.)

Answer 1

What you're calling a "predicate noun" is, in fact, the subject. In the Latin construction, unlike the English translation, the thing possessed is the subject, so the verb has to agree with it. E.g. in Puellis est rosa, even though this can be translated as "The girls have a rose", a literal translation would be something like "A rose is to/for the girls". The subject is rosa, so the verb agrees with it and is singular.

Answer 2

The thing being possessed is the subject in this construction. The verb agrees with the subject, but the subject in your example is not the girl. Do not confuse the plural nominative and singular dative, although they both end in -ae.

Consider these examples (cases indicated in parentheses):

• Girl has a rose. Puellae (dat) est rosa (nom).
• Girls have a rose. Puellis (dat) est rosa (nom).
• Girl has roses. Puellae (dat) sunt rosae (nom).
• Girls have roses. Puellis (dat) sunt rosae (nom).