Vigilia bona salutem urbis servat.

Because bona has to agree with vigilia.

• vigilia is feminine
• singular
• nominative (the subject form)

So the adjective bonus, bona, bonum becomes bona to match it.

This is a very important Latin rule: adjectives agree with the nouns they describe in gender, number, and case.

So:

• bonus = masculine nominative singular
• bona = feminine nominative singular
• bonam = feminine accusative singular

Since vigilia is feminine nominative singular, bona is the correct form.

We know because vigilia is in the nominative case, which is the normal case for the subject of a sentence.

Also, the verb is servat, which is third person singular, so it matches a singular subject.

So the sentence structure is:

• vigilia bona = the subject
• salutem urbis = the object phrase
• servat = the verb

In other words, good watchfulness / a good watch is doing the action of preserving.

Because salutem is the accusative singular form of salus.

The accusative is used for the direct object of the verb. In this sentence, the thing being preserved is salutem.

So:

• salus = nominative singular, safety
• salutem = accusative singular, safety as the direct object

Since servat means preserves / keeps / saves, it takes a direct object, and that object must be in the accusative.

Because urbis means of the city.

It is the genitive singular of urbs, urbis. The genitive often shows possession or a close relationship, and here it tells us whose safety is being preserved:

• salutem = safety
• urbis = of the city

Together, salutem urbis means the safety of the city.

This is a very common Latin pattern:

• amor patris = the love of the father
• porta urbis = the gate of the city
• salus urbis = the safety of the city

Servat is:

• third person
• singular
• present tense
• active voice
• indicative mood

It comes from servo, servare, servavi, servatus, meaning to save, preserve, protect, or keep.

So servat means:

• he/she/it preserves
• he/she/it protects
• he/she/it keeps safe

In this sentence, the subject is vigilia bona, so the natural translation is a good watch preserves or good watchfulness preserves.

Because Latin usually has no articles.

English often needs words like:

• the
• a
• an

But Latin normally does not have separate words for them. So vigilia can mean:

• a watch
• the watch
• sometimes just watchfulness

and salutem urbis can mean:

• the safety of the city
• a city’s safety, depending on context

You figure out whether to use the or a from the context and from normal English style.

Yes. Latin word order is much more flexible than English word order because the endings show the grammatical roles.

So this sentence could be rearranged in several ways without changing the basic meaning, for example:

• Vigilia bona salutem urbis servat.
• Salutem urbis vigilia bona servat.
• Urbis salutem vigilia bona servat.

The endings still show:

• vigilia = subject
• salutem = direct object
• urbis = genitive, of the city
• servat = verb

That said, word order can affect emphasis. The given order is perfectly natural and puts the verb at the end, which is very common in Latin.

Vigilia can have a few related meanings, depending on context, such as:

• wakefulness
• watchfulness
• a watch
• a guard duty
• a night watch

In this sentence, it most likely means something like watchfulness, careful vigilance, or a good watch.

A learner should know that Latin words often cover a range of meanings, and the best English translation depends on context.

Because urbis is a genitive noun, and genitives often attach naturally to a nearby noun to show of ....

Here the most sensible grouping is:

• salutem urbis = the safety of the city

That forms one noun phrase, while vigilia bona forms another:

• vigilia bona = good watchfulness / a good watch

So the sentence divides neatly into:

• subject: vigilia bona
• object: salutem urbis
• verb: servat

It describes vigilia, not salutem.

You can tell from the endings:

• vigilia = feminine nominative singular
• bona = feminine nominative singular

But:

• salutem = feminine accusative singular

If bona were describing salutem, it would have to be bonam, not bona.

So the grammar tells us clearly that bona modifies vigilia.

Here they are:

• vigilia, vigiliae = watchfulness, a watch, vigil
• bonus, bona, bonum = good
• salus, salutis = safety, welfare, health
• urbs, urbis = city
• servo, servare = preserve, save, protect

A beginner should get used to recognizing that the form in the sentence is often not the dictionary form:

• salutem comes from salus
• urbis comes from urbs
• servat comes from servo
• bona comes from bonus, bona, bonum

Because Latin very often places the verb near or at the end of the sentence.

This is not a strict rule, but it is extremely common. For learners coming from English, this can feel unusual, since English usually prefers:

• Subject + Verb + Object

But Latin often likes:

• Subject + Object + Verb

So:

• Vigilia bona salutem urbis servat

is a very normal Latin sentence shape.