December diem festum familiae adfert, et mense Decembri pueri domi libentius manent.

Because December is in the nominative case, which is the normal case for the subject of a verb.

In December diem festum familiae adfert, the verb is adfert = brings. So we ask, what brings the holiday to the family? The answer is December.

Latin often uses month names as nouns in this way, even though historically they are related to adjectives with an implied mensis (month).

Diem festum is accusative singular because it is the direct object of adfert.

• dies = day
• diem = accusative singular of dies
• festum agrees with diem and means festive or holiday

So literally, December brings a festive day / a holiday.

Familiae is in the dative singular because it is the indirect object: to the family.

The verb adferre means to bring to or to carry to, so it commonly takes:

• a direct object in the accusative: the thing being brought
• an indirect object in the dative: the person or group receiving it

So:

• diem festum = the holiday
• familiae = to the family

Adfert is the 3rd person singular present active indicative of adferre.

That means:

• 3rd person singular = he/she/it brings
• here the subject is December, so: December brings

You may also see the verb spelled affert. Both spellings are connected with the same verb:

• ad + ferre = to bring to
• the d often assimilates before f, producing affert

So adfert and affert are basically variant spellings of the same form.

Because mense Decembri is an ablative expression of time when, meaning in the month of December.

• mense = in the month / during the month (ablative of mensis)
• Decembri = ablative singular agreeing with mense

So the phrase means in December.

Latin can sometimes express time with just the ablative, but mense Decembri is a very clear and explicit way to say it.

Because the word is in a different case.

• December = nominative, used as the subject
• Decembri = ablative, used with mense in the time expression

Latin changes word endings to show grammatical function. English usually relies more on word order and prepositions; Latin often uses case endings instead.

Because domi is a special locative form meaning at home.

A few Latin words preserve a locative case, especially names of places and words like:

• domi = at home
• ruri = in the countryside
• humi = on the ground

So:

• domi manent = they stay at home

You can also see in domo in Latin, but domi is the usual idiomatic way to say at home.

Libentius is a comparative adverb, from libenter = willingly, gladly.

So:

• libenter = willingly
• libentius = more willingly, more gladly, more readily

It is an adverb because it modifies the verb manent (stay), not a noun.

So pueri domi libentius manent means:

• the boys stay at home more willingly or more naturally,
• the boys are more willing to stay at home

Yes. As a comparative adverb, libentius literally means more willingly. Latin often uses a comparative without stating the second half explicitly.

So the sentence implies something like:

• more willingly than usual
• more willingly than at other times
• more willingly than in other months

Latin does not have to spell out the comparison if the context makes it clear enough.

Because Latin word order is much freer than English word order.

Manent is at the end because Latin often places the verb late in the clause, especially in straightforward prose. But the endings already tell you the grammatical roles, so the sentence does not depend on English-style word order.

In the second clause:

• pueri = subject
• domi = at home
• libentius = more willingly
• manent = stay

Putting the verb last is very normal Latin style.

Because Latin has no articles.

English distinguishes:

• a holiday
• the holiday

Latin usually leaves that to context. So diem festum could be understood as:

• a holiday
• the holiday
• a festive day

The exact English translation depends on context, not on a separate Latin word for a or the.

Because pueri is nominative plural, so it means the boys and serves as the subject of manent.

• puer = boy
• pueri can mean either of the boy (genitive singular) or boys (nominative plural)

Here it must be nominative plural, because it is paired with manent, which is 3rd person plural = they stay.

So:

• pueri ... manent = the boys stay

Yes, two useful examples:

1. diem festum

   • diem is masculine accusative singular
   • festum matches it in gender, number, and case

2. mense Decembri

   • mense is ablative singular
   • Decembri matches it in case, number, and gender

This kind of agreement is one of the main things to watch for in Latin: adjectives must match the nouns they describe.