Decet bonos cives pacta servare et amicos non fallere.

Because decet often uses the person concerned in the accusative.

So Latin is structured like:

decet bonos cives ... = it is fitting for good citizens ...

That means bonos cives is not the nominative subject of decet. Instead, it is the person for whom the action is appropriate. It is also the understood subject of the infinitives servare and fallere: the good citizens are the ones keeping agreements and not deceiving friends.

Because bonos cives is not the grammatical subject of decet.

The sentence works more like:

It is fitting
for good citizens
to keep agreements and not deceive friends.

So the main idea being stated is the singular idea it is fitting, and that is why Latin has decet, not decent.

In natural English, we often turn this into good citizens should..., but Latin is built differently.

Here decet means it is fitting, it is proper, or it befits.

It commonly appears in a pattern like:

decet + accusative person + infinitive

So here:

• decet = it is fitting
• bonos cives = for good citizens
• pacta servare et amicos non fallere = to keep agreements and not deceive friends

So the whole sentence is a very typical moral statement.

Because they depend on decet and explain what is fitting.

• servare = to keep
• fallere = to deceive

Latin uses the infinitive after decet much as English uses to + verb:

• it is fitting ... to keep
• it is fitting ... not to deceive

Both infinitives are present active infinitives.

It is joining two whole infinitive phrases:

• pacta servare = to keep agreements
• amicos non fallere = not to deceive friends

So the structure is:

decet bonos cives [pacta servare] et [amicos non fallere].

In other words, et is linking two duties or actions, not just two objects.

Non negates only fallere.

So the sentence means:

• keep agreements
• and not deceive friends

It does not negate servare. The first action is positive; the second is negative.

Latin often places non directly before the word it is most closely negating, so non fallere is very clear.

Pacta is accusative plural neuter of pactum, meaning agreement, compact, or pact.

Here it is the direct object of servare:

• pacta servare = to keep agreements

Because it is neuter plural, its nominative and accusative plural forms look the same. In this sentence, it is accusative because it receives the action of servare.

Amicos is accusative plural masculine of amicus, and it is the direct object of fallere:

• amicos fallere = to deceive friends

That is different from bonos cives, which is also accusative, but for a different reason:

• bonos cives goes with decet as the people for whom the action is fitting
• amicos is simply the object of fallere

So both are accusative, but they do different jobs in the sentence.

Because agreement in Latin is based on gender, number, and case, not on declension.

• bonos = accusative plural masculine
• cives = accusative plural masculine

So they agree perfectly, even though:

• bonus is a 1st/2nd-declension adjective
• civis is a 3rd-declension noun

Latin adjectives can modify nouns from other declensions as long as the gender, number, and case match.

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

This sentence could be rearranged in other ways without changing the basic meaning, because the endings show how the words function. For example, Latin can move words around for emphasis.

The given order is natural and clear:

• decet comes first, setting the moral tone
• bonos cives follows as the people concerned
• then the two actions are given

So while the order is flexible, this arrangement is a good, idiomatic one.

That depends on how literal or natural you want to be.

Possible translations include:

• it is fitting for good citizens...
• it befits good citizens...
• it is proper for good citizens...
• good citizens should...
• good citizens ought to...

The more literal choices keep the flavor of fitness or propriety. The more natural English choices, like should or ought to, sound smoother but are a little less literal.