Bonos cives concordiae favere decet.

Because decet often works with an accusative + infinitive pattern.

In this sentence:

• bonos cives = the people for whom something is fitting
• concordiae favere = the action that is fitting

So the structure is essentially:

• bonos cives = it befits good citizens
• concordiae favere = to favor harmony

A very literal rendering would be:

• It is fitting for good citizens to favor harmony.

So even though good citizens may feel like the “subject” in English, in Latin they are actually in the accusative with decet.

The grammatical subject is the infinitive phrase:

• concordiae favere = to favor harmony

So the sentence is built like this:

• Bonos cives = accusative person affected
• concordiae favere = subject infinitive
• decet = is fitting / befits

In other words, Latin is saying:

• To favor harmony befits good citizens.

That sounds awkward in English, so we usually translate more naturally as:

• It is fitting for good citizens to favor harmony
• or Good citizens ought to favor harmony

Because favere takes the dative case, not a direct object in the accusative.

So:

• favere alicui = to favor someone/something
• concordiae favere = to favor harmony / concord

This is a very common point for English speakers, because English uses a direct object:

• favor harmony

But Latin uses the dative after faveo, favere:

• concordiae

So concordiae is not genitive here; it is dative singular.

The ending -ae can represent more than one case, especially in a first-declension noun, so context matters.

For concordia, concordiae:

• nominative singular: concordia
• genitive singular: concordiae
• dative singular: concordiae
• nominative plural: concordiae

Here, the verb favere strongly tells you what case to expect, because faveo governs the dative. So concordiae here must be:

• dative singular = to/for harmony, concord

favere is the present active infinitive of faveo, favere.

So:

• faveo = I favor
• favere = to favor

In this sentence, the infinitive functions as the thing that is fitting:

• concordiae favere decet = to favor harmony is fitting

Because faveo is one of those Latin verbs that governs the dative rather than the accusative.

English says:

• favor harmony

Latin says:

• concordiae favere

This is similar to other Latin verbs that take the dative, where English learners may expect a direct object. So it is best to learn faveo with its construction:

• faveo + dative

decet means something like:

• it is fitting
• it is proper
• it befits
• it is becoming

So the sentence has a moral or social sense of appropriateness:

• It is proper for good citizens to favor harmony.

Depending on context, English may translate this more idiomatically as:

• Good citizens ought to favor harmony.

But decet is not exactly the same as a plain future or a command; it expresses what is suitable or proper.

Yes, effectively it is.

The verb is third-person singular:

• decet = it is fitting / it befits

Here it is not saying that good citizens are the nominative subject. Instead, Latin uses:

• accusative person: bonos cives
• infinitive action: concordiae favere

So the sense is impersonal in English:

• It is fitting for good citizens to favor harmony.

Because bonus is an adjective modifying cives, and Latin adjectives agree with the nouns they describe in:

• gender
• number
• case

Here:

• cives = accusative plural
• bonos = accusative plural masculine

So:

• bonos cives = good citizens

Even though civis can be masculine or feminine depending on the person, in a general statement like this Latin often uses the masculine plural form.

cives comes from civis, civis, a third-declension noun.

Relevant forms are:

• nominative singular: civis
• accusative singular: civem
• nominative plural: cives
• accusative plural: cives

So cives could be either nominative plural or accusative plural by form alone. Here it is understood as accusative plural because of its role with decet, and because bonos clearly shows the accusative plural too.

Because cives has the same form in nominative plural and accusative plural, but bonos does not.

Compare:

• nominative plural masculine: boni
• accusative plural masculine: bonos

Since the sentence has bonos cives, the whole phrase must be accusative plural.

So even if cives by itself might be ambiguous, bonos removes the ambiguity.

Yes. Latin word order is relatively flexible because the endings show the grammatical relationships.

For example, these would express essentially the same basic meaning:

• Bonos cives concordiae favere decet.
• Concordiae favere bonos cives decet.
• Decet bonos cives concordiae favere.

The choice of order affects emphasis more than basic grammar.

In your sentence, putting bonos cives first gives prominence to good citizens as the people whose proper behavior is being described.

Yes, that is a very natural translation.

More literally, Latin says:

• It befits good citizens to favor harmony.

But in good English, that often becomes:

• Good citizens should favor harmony
• It is proper for good citizens to favor harmony
• Good citizens ought to favor harmony

The exact translation depends on how strongly you want to bring out the nuance of decet.

It can mean several closely related ideas, such as:

• harmony
• concord
• agreement
• unity

So concordiae favere can mean favoring social harmony, civic unity, or concord among people. The exact English word depends on context, but the core idea is peaceful agreement rather than conflict.