Limes inter duos agros servandus est.

Limes is nominative singular masculine. It is the subject of the sentence.

Its dictionary form is limes, limitis, a 3rd-declension noun. In this sentence, it is the thing that must be preserved.

Because the preposition inter takes the accusative case.

So:

• agros = accusative plural of ager
• duos = accusative plural masculine of duo

Together, inter duos agros means between the two fields.

Because duo is one of the Latin numerals that changes form to match gender, number, and case.

Here it must agree with agros, which is:

• masculine
• plural
• accusative

So the correct form is duos.

A few forms of duo are:

• masculine nominative: duo
• masculine accusative: duos
• feminine nominative/accusative: duae
• neuter nominative/accusative: duo

Servandus est is a passive periphrastic, formed from:

• the gerundive: servandus
• a form of sum: est

This construction expresses necessity, duty, or obligation.

So servandus est means something like:

• must be preserved
• is to be preserved
• has to be preserved

It is not just a simple passive like is preserved. It specifically adds the idea of necessity.

Because the gerundive servandus must agree with limes in:

• gender
• number
• case

Since limes is masculine singular nominative, the gerundive must also be masculine singular nominative:

• limes servandus est

If the subject were feminine singular, you would get servanda est.
If it were neuter singular, servandum est.
If it were masculine plural, servandi sunt.

Because the subject is not duos agros. The subject is limes, and limes is singular.

Duos agros is part of the prepositional phrase inter duos agros, which just tells you where or between what things the boundary is.

So the sentence is singular because there is one boundary being talked about.

In this sentence, Latin leaves the responsible person unstated.

That is very normal with the passive periphrastic. The sentence can simply express a general obligation: the boundary must be preserved.

If Latin wanted to say by whom it must be preserved, it could add a dative of agent, for example:

• agricolae limes servandus est = the boundary must be preserved by the farmer
• agricolis limes servandus est = the boundary must be preserved by the farmers

So the original sentence focuses on the rule or necessity, not on the person responsible.

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

So this sentence could be rearranged in other ways, for example:

• Inter duos agros limes servandus est
• Servandus est limes inter duos agros

These would still mean basically the same thing.

The given order is natural because:

• limes comes first as the topic
• inter duos agros explains which boundary
• servandus est comes at the end, where Latin often places the verbal idea

Yes. A sentence like Limes inter duos agros servari debet would also mean that the boundary must be preserved.

But there is a difference in feel:

• servandus est = a classic Latin way to express necessity, often compact and formal
• servari debet = also expresses obligation, but with debet plus infinitive

Both are possible, but the passive periphrastic is especially common in formal, legal, or rule-like statements.

Inter can mean both between and among, depending on context.

For example:

• with two things, it is naturally between
• with more than two, it can mean among

In this sentence, because of duos agros, the meaning is clearly between the two fields.

So inter does not itself force the idea of exactly two; the numeral duos does that.

Limes can mean a boundary, border, limit, or even a frontier. In some contexts it can also refer to a boundary path or strip of land marking a division.

So in this sentence, it refers to the boundary line or border separating the two fields.

This is a good example of how Latin words often have a range of meanings, and the exact sense depends on context.