Post cenam omnes in peristylio sedent et de die longo loquuntur.

Because post is a preposition that takes the accusative case when it means after.

• cena = dinner, meal
• cenam = accusative singular of cena

So:

• post cenam = after dinner

This is something you simply learn with the preposition: post + accusative.

Omnes is nominative plural, and it means all or everyone.

Here it is the subject of both verbs:

• omnes sedent = everyone sits / all are sitting
• omnes loquuntur = everyone speaks / all are talking

Because the subject is plural, the verbs are also plural.

Because in with the ablative usually shows location: in, on, or at a place.

• in peristylio = in the peristyle / courtyard

Compare the two common uses of in:

• in + ablative = where?
  
  • in peristylio = in the courtyard
• in + accusative = into where? toward where?
  
  • in peristylium = into the courtyard

So in this sentence, they are already there, which is why Latin uses the ablative.

Peristylio is the ablative singular of peristylium, a neuter noun.

A peristylium is a colonnaded courtyard or inner open area of a Roman house. In many beginner Latin readings, it is often translated simply as courtyard.

So:

• peristylium = courtyard / peristyle
• in peristylio = in the courtyard

Sedent comes from sedere, meaning to sit.

It is:

• present tense
• third person plural

So it means:

• they sit
• or, in natural English, they are sitting

It matches the plural subject omnes.

Because loquor, loqui, locutus sum is a deponent verb.

A deponent verb:

• has passive-looking forms
• but an active meaning

So loquuntur looks like a passive ending, but it means:

• they speak
• they are talking

This is completely normal for deponent verbs. You should learn loquor as a deponent from the start.

Because de takes the ablative case.

So:

• dies = day
• die = ablative singular
• longo agrees with die in case, number, and gender

Therefore:

• de die longo = about the long day

A good rule to remember is:

• de + ablative

Good question. Dies is usually masculine, especially when it just means day in an ordinary sense.

So here:

• de die longo uses masculine longo

You may sometimes see dies as feminine in certain special contexts, but for most beginner sentences, treat dies as masculine unless you have a reason not to.

Latin word order is much more flexible than English word order. Both of these could make sense:

• de die longo
• de longo die

Latin often places words in the order that sounds natural or emphasizes certain parts of the sentence, rather than following a fixed English-style pattern.

So die longo simply means long day, with the adjective longo agreeing with die.

Because Latin often prefers to place the verb near the end, especially in straightforward narrative prose.

The sentence structure is:

• Post cenam = after dinner
• omnes = everyone
• in peristylio = in the courtyard
• sedent = sit
• et = and
• de die longo = about the long day
• loquuntur = talk

This end-placement of verbs is very common in Latin, though Latin word order is flexible and not completely fixed.

Yes. In this sentence, de means about or concerning.

So:

• de die longo loquuntur = they talk about the long day

Depending on context, de can also mean things like from, down from, or concerning, but about is the natural meaning here.

Yes. Omnes can mean:

• all
• all the people
• everyone

The exact English translation depends on context. Since there is no noun stated here, English often translates it smoothly as everyone.

So omnes sedent is naturally:

• everyone is sitting or
• all are sitting

It is present tense, and in context it can mean either:

• a general present: they sit and talk
• or a present progressive idea: they are sitting and talking

Latin does not have a separate form exactly like English are sitting or are talking, so the simple present often covers both ideas. Context determines which English version sounds best.