Today's introit is taken from the third mass of Christmas (during the day). The first part is taken from the Book of Isaias chapter 9. The second part is from the opening verse of Psalm 97.
Puer natus est nobis, et Filius datus est nobis; cujus imperium super humerum ejus: et vocabitur nomen ejus magni Consilii Angelus. * Cantate Domino canticum novum, quia mirabilia fecit.
A Child is born unto us, and a Son is given to us; and the government is upon his Shoulder: and his name shall be called the Angel of the great Counsel. * Sing to the Lord a new Canticle, for he hath done wonderful things.
Puer - A boy. The subject of the verb natus est.
natus est - He has been born. Nascor/nasci/natus is the verb - to be born. Here we have the perfect tense, indicating something that has happened in the past.
nobis - for us. Dative case, indicating something done for our benefit.
et - and
Filius - A son. Subject of the verb datus est.
datus est - he has been given. The verb is do/dare/dedi/datum - to give. This is the perfect passive, indicating something that happened in the past. It is passive because the subject has had something done to it - being given - rather than having done something.
nobis - for us. As above.
cujus - of whom/whose.
imperium - government/empire.
In English we would put the verb 'is' in this position. In Latin, it is often omitted, like it is here.
super - upon or above. Takes the the accusative here, since there is an indication of movement - the government is moving onto his shoulders.
humerum - shoulder/upper arm. Humerus is the noun. The changed ending indicates the accusative case, to follow super.
ejus - his.
et - and.
vocabitur - it will be called. voco/vocare/vocavi/vocatum - to call. The ending indicates future and passive voice - something that will happen in the future to the subject, his name.
nomen - name. Subject of the verb vocabitur.
ejus - his.
magni - of great. Magnus is great. The ending indicates the genitive case, to agree with Consilii, which it modifies.
Consilii - of Counsel. Consilium is counsel. The ending indicates the genitive case, for possession - of counsel.
Angelus - the Angel. This is nominative case. Strictly speaking, this isn't the object of the verb vocabitur, but its complement. See the grammatical note below.
Cantate - Sing! The imperative plural, indicating a command given to several people. The verb is canto/cantare/cantavi/cantatum - I sing.
Domino - to the Lord. Dative case of Dominus, the Lord.
canticum - a song. Accusative case, as the direct object of the verb cantate.
novum - new. Novus is new. The ending indicates accusative, to agree with the noun canticum which it modifies.
quia - for/since.
mirabilia - wonderful things. Mirabilis is wonderful. Here the ending indicates plurality - wonderful things. This is the direct object of the verb fecit.
fecit - he has done. The verb facio/facere/feci/factum means to do or to make. Here we have the perfect tense, indicating something that has been done.
1. Complement of verbs.
Most verbs have a subject, which appears in the nominative case, and possibly a direct object, which appears in the accusative case.
Some verbs, called copulative verbs, join together two two expressions, both of which are in the nominative case. These verbs typically are expressing some sort of identity between the two expressions, and the most obvious is the verb 'to be' -
e.g. Johannes est prophetus magnus.
John is a great prophet.
Here, both Johannes (John) and prophetus magnus (great prophet) are in the nominative case.
Another verb that is copulative in the same way is vocari - to be called. This was seen in today's introit:
Vocabitur nomen ejus magni consilii angelus.
Two expressions - nomen ejus (his name) and magni consilii angelus (the angel of great counsel) - are joined together by the copulative verb vocabitur (it will be called), and both expressions are in the nominative case.
(Reference: Kennedy's Revised Latin Primer, s.185-189)
Prepositions indicate the relation of a noun, adjective or pronoun to other words in the sentence. For example, in English: in, through, over, below.
super - above
sub - below
in - in or into
ad - towards
ex - out of
de - concerning
Latin prepositions govern either the accusative or ablative cases. This means that the noun following on from one of them is in the case that is governed by the preposition.
Ad governs the accusative.
Towards Rome is Ad Romam. Romam is in the accusative case, since it follows ad.
Ex governs the ablative.
Out of Rome is Ex Roma. Roma is in the ablative case, since it follows ex.
(Usually this would be E Roma rather than Ex Roma - ex becomes e before a consonant).
In, super, sub, and subter - these four prepositions can govern either the accusative or ablative. They govern the accusative if there is a sense of motion; the ablative if there is a sense of fixed state.
Into Rome is In Romam. Accusative.
In Rome (fixed location) is In Roma. Ablative.
(Reference: Kennedy's Revised Latin Primer, s.171-174)