Pater dicit aerem ruri puriorem esse quam in suburbio.

Because after dicit Latin is using an indirect statement.

With verbs like dico, puto, scio, audio, Latin often says:

• verb of saying/thinking
• • accusative subject
• • infinitive

So:

• Pater dicit = Father says
• aerem ... esse = the air to be ...

In a direct statement, you would have:

• Aer ruri purior est quam in suburbio.

But once it becomes reported after dicit, the subject aer changes to the accusative aerem, and est changes to the infinitive esse.

Esse is the infinitive that completes the indirect statement.

Latin does not usually say Father says that the air is purer... with a separate word for that. Instead, it says literally:

• Father says the air to be purer...

So esse is the equivalent of the English finite verb is inside reported speech.

A helpful way to see it:

• direct: Aer ... purior est
• indirect after dicit: aerem ... puriorem esse

Because puriorem agrees with aerem, which is accusative singular.

Even though puriorem is translated with English purer, it is still an adjective, and Latin adjectives must agree with the noun they describe in case, number, and gender.

Here:

• aerem = accusative singular masculine
• puriorem = accusative singular masculine/feminine comparative

So puriorem is the correct form.

If the clause were direct, you would get:

• Aer ... purior est

But in the indirect statement, both the noun and its predicate adjective shift into the accusative:

• aerem ... puriorem esse

It is an adjective, not an adverb.

It describes aerem: the air is purer.

That is why it agrees with aerem in case.

If Latin wanted an adverb like more purely, it would use a different form. But that is not what is happening here. The sentence is talking about the quality of the air, not the manner of an action.

Because classical Latin usually does not use a separate that-clause after verbs of saying, thinking, knowing, and perceiving. Instead, it uses the accusative-and-infinitive construction.

So English says:

• Father says that the air is purer...

But Latin says:

• Pater dicit aerem ... esse

This is one of the most important patterns in Latin syntax.

Because ruri is a locative form, meaning in the countryside.

The locative is an old case used mainly for place where. In Latin it survives in a limited number of words, especially names of towns and a few common nouns.

So ruri already means:

• in the country
• in the countryside

That is why Latin does not need in here.

Ruri is the locative singular of rus, ruris, meaning the countryside or the country.

This is a useful word to remember because it is a little unusual:

• dictionary form: rus, ruris
• locative: ruri = in the country

So in this sentence, ruri is not dative or ablative in meaning; it is specifically a locative expression.

Because the two nouns behave differently.

• rus has a special locative form: ruri
• suburbium does not normally use a locative here, so Latin uses in + ablative

Thus:

• ruri = in the country
• in suburbio = in the suburb

So the difference is grammatical, not a difference in meaning.

Suburbio is ablative singular after in.

With in meaning in/on for location, Latin normally takes the ablative:

• in suburbio = in the suburb

If in meant motion into something, it would take the accusative instead. But here it is location, so the ablative is correct.

Quam introduces the second part of the comparison: than.

So:

• puriorem ... quam in suburbio
• purer ... than in the suburb

Latin comparatives can be expressed in two common ways:

1. comparative + quam
2. comparative + ablative of comparison

Here quam is the natural choice, because the second item is not just a simple noun but the phrase in suburbio.

Because Latin often omits words that are easy to understand from the context.

After quam in suburbio, you can mentally supply something like:

• [est]
• or, in the indirect statement framework, the idea already contained in esse

In other words, Latin does not need to repeat the whole thought. The meaning is simply:

• the air to be purer in the country than in the suburb

English does something similar sometimes: The air is cleaner here than in the city. We do not repeat it is at the end.

Because Latin has no articles.

There is no separate word for the or a/an in classical Latin. Whether you translate a noun as air, the air, or an air depends on context.

So:

• aerem can mean air or the air
• suburbio can mean a suburb or the suburb

English must choose an article, but Latin does not have to.

The word order is meaningful, but not in the same rigid way as English.

Latin word order is flexible because the endings show the grammatical relationships. This sentence is arranged quite naturally:

• Pater dicit — main statement first
• aerem ... esse — the indirect statement
• ruri and in suburbio — the places being compared
• puriorem close to esse — the key idea of the reported statement

A very literal English order would sound odd, but in Latin this order is perfectly normal. The endings, not the position alone, tell you what each word is doing.