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.

So:

• post = after
• cenam = accusative singular of cena (dinner, meal)

This is just something you learn with the preposition:
post + accusative = after ...

So post cenam means after dinner.

Omnes is the nominative plural form of omnis, meaning all or everyone.

Here it is being used substantively, which means the adjective is standing on its own like a noun:

• omnes = all (the people) / everyone

Latin does this very often. English does too sometimes, as in the rich, the poor, the brave.

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

• sedent = they sit / are sitting
• loquuntur = they speak / are speaking

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

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

Here the people are already sitting in the peristyle, so this is location, not motion:

• in peristylio = in the peristyle

If the sentence were talking about going into it, you would expect the accusative:

• in peristylium intrant = they enter into the peristyle

Peristylio is ablative singular.

The dictionary form is peristylium, a second-declension neuter noun. Its ablative singular ends in -o:

• nominative: peristylium
• ablative: peristylio

It is ablative here because it follows in in the sense of location.

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

So when the sentence says in peristylio, it places the scene in that part of the house.

Sedent is:

• 3rd person plural
• present tense
• active voice
• indicative mood

It comes from sedeo, sedere, meaning sit or be sitting.

So sedent can be translated as:

• they sit
• they are sitting

Latin present tense often covers both simple present and present progressive English meanings.

Because loquor is a deponent verb.

A deponent verb has:

• passive-looking forms
• but an active meaning

So:

• loquor = I speak
• loqueris = you speak
• loquitur = he/she speaks
• loquuntur = they speak

Even though -ntur often looks passive, here it is not translated passively.
So loquuntur means they speak or they are talking, not they are spoken.

Because de takes the ablative case.

Here de means about or concerning, so:

• de = about
• die = ablative singular of dies (day)
• longo = ablative singular of longus (long), agreeing with die

So de die longo literally means about the long day.

Because dies is a fifth-declension noun, and its ablative singular is die.

A few common forms are:

• nominative singular: dies = day
• accusative singular: diem
• ablative singular: die

Since de requires the ablative, Latin uses die.

Longo is ablative singular and agrees with die.

The noun dies is often masculine, especially in the ordinary sense of day, so the adjective is masculine too:

• die longo = on/about the long day

Agreement in Latin means the adjective matches the noun in:

• case
• number
• gender

So since die is ablative singular masculine here, longo is too.

Because Latin word order is much more flexible than English word order.

This sentence is arranged in a very natural Latin way:

• Post cenam = time expression first
• omnes = subject
• in peristylio = place
• sedent et de die longo loquuntur = the actions

Latin often places verbs later in the sentence, especially in straightforward narrative or description. But other orders would also be possible if the writer wanted a different emphasis.

Yes. Omnes is the subject of both verbs.

So the structure is:

• omnes sedent
• omnes loquuntur

Latin often gives the subject once and then uses it for multiple verbs joined by et.

Yes. Latin does not usually make the same sharp distinction that English does between:

• they sit
• they are sitting

The present tense sedent can cover both, and the context tells you which English wording sounds most natural.

The same is true for loquuntur:

• they speak
• they are speaking
• they talk
• they are talking

All can be possible depending on context.

Because de + ablative is a very common Latin way to express the topic of speech or thought.

So:

• de amico loqui = to speak about a friend
• de bello scribere = to write about war
• de die longo loquuntur = they talk about the long day

The basic physical idea of de is something like down from or concerning/from the subject of, but in many sentences it simply means about.

Yes. Omnes in this form can be:

• masculine plural
• or feminine plural

If the group is mixed or unspecified, Latin often uses the masculine plural. But without more context, omnes could simply mean everyone regardless of gender.