In altera historia de regno antiquo parvus exercitus urbem servat.

Because in can take two different cases:

• in + ablative = in / on with no movement
• in + accusative = into / onto with movement toward something

Here the idea is in another story or in the other story, not into another story, so Latin uses the ablative: in altera historia.

It actually is in the ablative singular, but for many first-declension nouns, the nominative singular and ablative singular can look the same if macrons are not written.

So:

• nominative: historia
• ablative: historiā

Without macrons, both appear as historia.
The preposition in tells you that this must be ablative here.

Altera means the other or another, often with the idea of the second of two.

It agrees with historia, so it is:

• feminine
• singular
• ablative

That is why it appears as altera.

A learner may wonder why Latin does not use alia here. The difference is:

• alia = another, different, other in a general sense
• altera = the other one, another of two

In many simple classroom sentences, altera is often translated naturally as another.

Because the preposition de takes the ablative case.

So:

• regnum becomes regno
• antiquus becomes antiquo to agree with regno

That gives:

• de regno antiquo = about the ancient kingdom

You can tell by agreement.

Latin adjectives agree with the nouns they describe in:

• gender
• number
• case

Here:

• regno is neuter, singular, ablative
• antiquo is also neuter, singular, ablative

But historia is feminine, singular, ablative, so an adjective describing it would need a feminine form, not antiquo.

So antiquo must describe regno.

Because parvus exercitus is in the nominative singular, which is the normal case for the subject.

Also, servat is third person singular, so it matches a singular subject:

• parvus exercitus = the small army
• servat = protects / saves

So the small army is the one doing the action.

Yes, it can look confusing at first.

Many learners expect a noun ending in -us to be second declension, but exercitus is actually a fourth-declension noun.

Its dictionary form is:

• exercitus, exercitūs = army

Its nominative singular is exercitus, so the form in the sentence is perfectly normal as the subject.

Because urbem is the accusative singular, used for the direct object.

The word urbs means city, but its forms are:

• nominative singular: urbs
• accusative singular: urbem

Since the city is receiving the action of servat, it must be in the accusative:

• urbem servat = protects the city

Servat is:

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

It comes from servare, meaning to save, to protect, or to keep safe.

So servat means:

• he/she/it protects
• he/she/it saves

In this sentence, the subject is parvus exercitus, so the sense is the small army protects the city.

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

In this sentence:

• parvus exercitus = subject
• urbem = object
• servat = verb

English usually prefers subject + verb + object:

• The small army protects the city

But Latin often puts the verb at the end:

• parvus exercitus urbem servat

That word order is very common and natural in Latin.

Latin has no definite or indefinite article.

So a word like urbem could mean:

• the city
• a city

And parvus exercitus could mean:

• the small army
• a small army

The exact English article depends on context.
That is why the same Latin sentence can sometimes be translated slightly differently in English.