In stabulo una vacca et unus vitulus sunt.

Because in takes the ablative when it means in or inside in a stationary sense.

• in stabulo = in the stable / in the barn
• If there were motion into something, Latin would usually use in with the accusative instead.

So:

• in stabulo = in the stable
• in stabulum = into the stable

Because unus, una, unum changes form to agree with the noun it describes.

Here:

• vacca is feminine, so Latin uses una
• vitulus is masculine, so Latin uses unus

This is called agreement: the numeral/adjective must match the noun in gender, number, and case.

So:

• una vacca = one cow
• unus vitulus = one calf

They are both in the nominative singular because they are the subjects of the sentence.

The sentence is saying that a cow and a calf are somewhere. Since they are the things doing the being, they are subjects.

So:

• vacca = nominative singular
• vitulus = nominative singular

Because together they make a compound subject.

Even though each noun is singular by itself, a cow and a calf means two things, so the verb must be plural:

• est = is
• sunt = are

That is why Latin uses sunt, not est.

Latin word order is much more flexible than English word order because the endings show the grammatical function of each word.

Putting the verb at the end is very common in Latin, especially in simple textbook sentences.

So this sentence could also be rearranged without changing the basic meaning, for example:

• Una vacca et unus vitulus in stabulo sunt.
• In stabulo sunt una vacca et unus vitulus.

The original order is just a normal and natural Latin way to say it.

It most literally means one.

Latin does not have articles like English a/an and the, so a bare noun like vacca can sometimes mean a cow or the cow, depending on context.

By adding una and unus, the sentence makes the number explicit:

• una vacca = one cow
• unus vitulus = one calf

In many contexts, English may translate this naturally as a cow and a calf, but Latin is clearly marking one of each.

Because Classical Latin has no articles.

English usually needs articles:

• a cow
• the cow

Latin normally just uses the noun by itself, and the reader understands from context whether it is indefinite or definite.

So:

• vacca could mean a cow or the cow
• vitulus could mean a calf or the calf

If Latin wants to stress one, it can use una/unus, as it does here.

Yes. It behaves like an adjective even though it is also a numeral.

It describes the noun and agrees with it:

• una vacca
• unus vitulus

So a learner can think of unus, una, unum as the Latin word for one, but grammatically it works a lot like an adjective.

Yes. Latin could simply say:

• In stabulo vacca et vitulus sunt.

That would still mean something like There are a cow and a calf in the stable or The cow and the calf are in the stable, depending on context.

Adding una and unus makes the counting clearer: one cow and one calf.

Et means and. It joins the two subjects:

• una vacca
• unus vitulus

So the subject is:

• una vacca et unus vitulus = one cow and one calf

Because et joins them into a pair, the verb becomes plural: sunt.

Yes. Latin often allows flexibility in adjective placement.

For example, you might also see:

• vacca una
• vitulus unus

In this sentence, una vacca and unus vitulus are straightforward and very common for beginners. Putting una/unus first can also make the number stand out a little more clearly.

Not always exactly, but it is close.

Stabulum can refer to a place where animals are kept, such as a stable, stall, or sometimes more generally a barn-like shelter depending on context.

So in stabulo is best understood as in the stable or in the barn, depending on the situation.

Because Latin can simply use the verb to be without adding a dummy subject like English there.

English says:

• There are a cow and a calf in the stable.

Latin just says:

• In stabulo una vacca et unus vitulus sunt.

The idea of existence is understood from sunt plus the nouns. Latin does not need a separate word corresponding to English there in this kind of sentence.