Avunculus cum nepote in horto sedet, dum matertera cum nepte in atrio loquitur.

Latin often uses more precise family terms than English.

• avunculus = a mother’s brother
• matertera = a mother’s sister

So these are specifically maternal relatives.

For comparison:

• patruus = a father’s brother
• amita = a father’s sister

English usually just says uncle and aunt, but Latin can be more exact.

These come from:

• nepos → nepote
• neptis → nepte

They can mean:

• grandson / granddaughter
• sometimes nephew / niece, depending on context

So Latin nepos and neptis are a little broader than the usual English words. The exact meaning comes from context.

Because they are part of phrases with cum:

• cum nepote = with the grandson/nephew
• cum nepte = with the granddaughter/niece

The actual subjects are:

• avunculus for sedet
• matertera for loquitur

So the sentence means that the uncle is sitting, and the aunt is speaking, while the younger relatives are simply with them.

Because cum meaning with takes the ablative case.

So:

• nepos → ablative singular nepote
• neptis → ablative singular nepte

This is a very common Latin pattern:

• cum amico = with a friend
• cum puella = with a girl
• cum nepote = with the grandson/nephew

They are in the ablative case because in is being used to mean in or inside, showing location.

• in horto = in the garden
• in atrio = in the atrium

When in shows place where, it takes the ablative.

Compare:

• in horto = in the garden
• in hortum = into the garden

So:

• ablative = location
• accusative = motion toward

They introduce different kinds of information:

• cum + ablative = with someone
• in + ablative = in a place

So here:

• cum nepote = with the grandson/nephew
• in horto = in the garden
• cum nepte = with the granddaughter/niece
• in atrio = in the atrium

Even though both use the ablative here, they do different jobs.

dum here means while.

It connects the two parts of the sentence:

• Avunculus cum nepote in horto sedet
• dum matertera cum nepte in atrio loquitur

So the sense is:

• The uncle sits in the garden with the grandson/nephew, while the aunt speaks in the atrium with the granddaughter/niece.

It shows that the two actions are happening at the same time.

Because loquitur comes from loquor, a deponent verb.

A deponent verb:

• has passive forms
• but an active meaning

So:

• loquitur looks passive in form
• but means he/she speaks, is speaking, or talks

This is completely normal in Latin. Other common deponent verbs include:

• sequitur = follows
• moritur = dies
• utitur = uses

Because the subject of each verb is still just one person.

• avunculus ... sedet = the uncle sits
• matertera ... loquitur = the aunt speaks

The phrases with cum do not create a compound subject. They just add companions:

• with the grandson/nephew
• with the granddaughter/niece

If Latin wanted to say the uncle and the grandson sit, it would use a true compound subject, not just cum.

The word order is fairly natural, but Latin word order is generally more flexible than English word order.

This sentence is arranged neatly:

• subject first: Avunculus
• accompaniment: cum nepote
• place: in horto
• verb: sedet
• then the second clause with dum
• subject first again: matertera
• accompaniment: cum nepte
• place: in atrio
• verb: loquitur

Latin often places the verb near the end, which happens here in both clauses. But other orders would still be possible if the writer wanted a different emphasis.

Because Latin often expresses location simply with a main verb plus a prepositional phrase.

So:

• sedet already means sits
• in horto tells you where

Together:

• in horto sedet = sits in the garden

Latin does not need an extra verb like is here. English sometimes does something similar too: He sits in the garden.

Here is the basic grammar of the nouns in this sentence:

• avunculus: nominative singular, masculine
  
  • the subject of sedet
• matertera: nominative singular, feminine
  
  • the subject of loquitur
• nepote: ablative singular
  
  • used after cum
• nepte: ablative singular
  
  • used after cum
• horto: ablative singular
  
  • used after in for location
• atrio: ablative singular
  
  • used after in for location

So one useful way to read the sentence is:

• nominative = who is doing the action
• ablative with cum = with whom
• ablative with in = where

Yes. loquor, loqui often means:

• speak
• talk
• sometimes converse

In this sentence, because it is followed by cum nepte, the sense may feel especially like talking with someone rather than giving a speech.

So natural English translations include:

• speaks
• is speaking
• talks
• is talking

The exact English wording depends on style more than grammar.

Classical Latin has no definite article like English the, and no indefinite article like a/an either.

So:

• avunculus can mean the uncle or an uncle
• horto in context can mean in the garden or in a garden

English has to choose one when translating, but Latin usually leaves that to context.