Cum ad villam reditur, de columbis et passeribus diu disputatur.

Because this cum is a conjunction, not the preposition.

• As a preposition, cum means with and is followed by the ablative:
  cum amico = with a friend
• As a conjunction, cum means when, since, or although, depending on context.

In Cum ad villam reditur, it introduces a time clause, so it means when.

Reditur is an impersonal passive form.

It is:

• from redeo, redire = to return
• present passive indicative, 3rd person singular

Because redeo is an intransitive verb, it does not normally take a direct object. In Latin, many intransitive verbs can form an impersonal passive, where the verb appears in the 3rd singular passive and means something like:

• people return
• one returns
• there is a return

So ad villam reditur does not mean the house is returned. It means people return to the house/home.

For the same reason: it is another impersonal passive.

Disputatur is:

• from disputo, disputare = to discuss / argue / debate
• present passive indicative, 3rd singular

In impersonal use, Latin keeps the verb in the 3rd person singular passive, even if several people are involved in the action.

So:

• de columbis et passeribus disputatur = there is discussion about doves and sparrows
• more naturally in English: people discuss doves and sparrows

Latin often uses the impersonal passive when the doer is general, unspecified, or unimportant.

So the sentence does not focus on who returns or who argues. It presents the actions more generally:

• reditur = people return / there is returning
• disputatur = people discuss / there is discussion

If the author wanted to name or emphasize the people doing the actions, active forms such as redeunt and disputant would be more likely.

Because ad takes the accusative and expresses motion toward a place.

So:

• ad = to, toward
• villam = accusative singular of villa

Together:

• ad villam = to the house / to the villa / toward the country house

This is the normal construction for movement toward something.

Because de means about, concerning here, and de takes the ablative.

So:

• columbis = ablative plural of columba
• passeribus = ablative plural of passer

Therefore:

• de columbis et passeribus = about doves and sparrows

This is a very common way in Latin to say what a discussion is about.

They are in different cases because different prepositions require different cases.

• villam = accusative singular
  because ad takes the accusative
• columbis = ablative plural
  because de takes the ablative
• passeribus = ablative plural
  because de takes the ablative

So the sentence shows two very common patterns:

• ad + accusative = motion toward
• de + ablative = about / concerning

Diu is an adverb meaning for a long time or a long while.

It modifies disputatur:

• de columbis et passeribus diu disputatur
  = people talk for a long time about doves and sparrows

It tells us how long the discussion goes on.

Both are present indicative passive, 3rd person singular.

So grammatically they are:

• reditur = it is returned / people return
• disputatur = it is discussed / people discuss

In context, the present tense can express:

• a general truth
• a habitual action
• a vivid present-time scene

Here it most naturally sounds like a general or repeated situation: when people return to the villa, they discuss doves and sparrows for a long time.

Because this is a straightforward temporal clause: when people return to the villa...

Latin often uses:

• cum + indicative for simple time: when
• cum + subjunctive for more circumstantial, causal, or concessive meanings, such as since or although, or for a more developed background idea

Here the clause is simple and factual, so the indicative is expected:

• Cum ad villam reditur = When people return to the villa

Not in the ordinary English sense of a passive.

Since redeo is intransitive, it cannot have a normal personal passive like the villa is returned. Instead, Latin uses the impersonal passive:

• reditur = people return / there is returning

So it is morphologically passive, but functionally it is an idiomatic way to express an indefinite subject.

It is formally a passive, but here it works as an impersonal expression.

A very literal rendering would be:

• it is discussed about doves and sparrows

But English does not say that naturally. So we translate the sense:

• people discuss doves and sparrows
• there is discussion about doves and sparrows

This is a good example of how Latin and English organize ideas differently.

Latin word order is much freer than English word order, because the endings show the grammatical relationships.

This sentence is arranged quite naturally in Latin:

• Cum ad villam reditur
  first the subordinate clause
• de columbis et passeribus
  then the topic of discussion
• diu
  then the adverb
• disputatur
  with the verb at the end

Placing the verbs at the ends of their clauses is especially common in Latin prose.

Yes. Villa can mean several related things depending on context:

• house
• country house
• estate
• farmhouse

So ad villam may mean:

• to the house
• to the villa
• back to the country house / estate

The exact English choice depends on the context, but the grammar stays the same.

Yes, but it is indefinite, not named.

With the impersonal passive, the implied subject is something like:

• people
• they
• one

So the sentence does not tell us exactly who is returning and discussing. It leaves that general or understood from context. That is one of the main functions of this construction in Latin.