Post cēnam omnēs in peristyliō sedent et dē diē longō loquuntur.

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

• post cēnam = after dinner
• cēna is the nominative form
• cēnam is the accusative singular

So this is a very common pattern:

• post cēnam = after dinner
• post bellum = after the war
• post multōs annōs = after many years

Omnēs means everyone or all here.

Grammatically, it is:

• nominative plural
• masculine or feminine, depending on the people being referred to

In this sentence, omnēs is the subject of both verbs:

• omnēs sedent = everyone sits / they all sit
• omnēs loquuntur = everyone speaks / they all talk

Even though English often says everyone with a singular verb, Latin can use a plural idea here: all the people.

Because in can take either the ablative or the accusative, depending on the meaning.

• in + ablative = in / on a place, showing location
• in + accusative = into / onto a place, showing motion toward

Here the people are already sitting in the peristyle, so it is location:

• in peristyliō = in the peristyle

Compare:

• in peristyliō sedent = they are sitting in the peristyle
• in peristylium intrant = they enter into the peristyle

A peristylium is a peristyle, a courtyard or open central area in a Roman house, usually surrounded by columns.

For a learner, the important thing is mostly grammatical:

• peristylium is a neuter noun
• here it appears in the ablative singular: peristyliō
• after in with location, that gives in peristyliō

Sedent is:

• present tense
• 3rd person plural
• from sedēre = to sit

So it means:

• they sit
• or more naturally in English here, they are sitting

Latin present tense often covers both simple present and present progressive:

• sedent can mean they sit or they are sitting

Because loquor, loquī, locūtus sum is a deponent verb.

Deponent verbs:

• have passive forms
• but active meanings

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

• they speak
• they talk

This is very important:

• loquitur = he/she speaks
• loquuntur = they speak

It does not mean they are spoken.

Dē takes the ablative case.

So:

• dē = about / concerning / from / down from, depending on context
• diē is ablative singular of diēs
• longō is ablative singular agreeing with diē

Together:

• dē diē longō = about the long day

This is a normal pattern:

• dē amīcō = about a friend
• dē rē pūblicā = about the republic
• dē diē longō = about the long day

In Latin, word order is much more flexible than in English.

Both of these can mean the same thing:

• diē longō
• longō diē

The adjective longō agrees with diē in:

• case: ablative
• number: singular
• gender: masculine

So the relationship is shown by the endings, not by fixed word order.

Latin authors often move words around for emphasis, rhythm, or style. A learner should focus first on the endings, not just the position.

Yes. Latin very often puts the verb at the end of the clause or sentence, although it does not have to.

So this word order is very natural:

• Post cēnam omnēs in peristyliō sedent et dē diē longō loquuntur.

A more literal word-by-word arrangement in English might feel like:

• After dinner everyone in the peristyle sit and about the long day talk.

That sounds odd in English, but it is normal in Latin. The main point is:

• Latin word order is flexible
• the verb often comes last
• the endings tell you how the words fit together

Because one subject, omnēs, is doing two actions:

• sedent = they are sitting
• loquuntur = they are talking

These two verbs are linked by et = and.

So the structure is:

• omnēs sedent et loquuntur = everyone is sitting and talking

This is completely normal in Latin, just as in English:

• The boys sit and listen
• The guests eat and laugh

In this sentence, no. Here post clearly means after in the sense of time.

Latin post can be:

• a preposition meaning after
• sometimes an adverb meaning afterward or behind, depending on context

But with cēnam and the context of daily activity, post cēnam naturally means:

• after dinner

A spatial meaning like behind the dinner would not make sense here.

The dictionary form is diēs.

More fully:

• diēs, diēī = day

In the sentence, diē is the ablative singular because it follows dē.

So:

• dictionary form: diēs
• sentence form: diē

This is a useful reminder that the form you see in a sentence is often not the dictionary form.