Learn Hardcore Latin: Vigilia bona salutem urbis servat. - Good watchfulness preserves the safety of the city.

Breakdown of Vigilia bona salutem urbis servat.

Why is bona used, and not bonus or bonam?

Because bona has to agree with vigilia.

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:

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

How do we know that vigilia bona is the subject?

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:

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