In mensa ampulla aquae et vas parvum stant.

Because in mensa shows location: on/in the table.

After in, Latin usually works like this:

• in + ablative = in/on somewhere, with no movement
• in + accusative = into/onto somewhere, with movement

Here nothing is moving. The bottle and the small vase are already there, so Latin uses the ablative: in mensa.

Because Latin in with the ablative can cover both English in and on, depending on the noun and the situation.

With mensa (table), natural English usually says on the table, even though Latin says in mensa. So this is a normal difference between the languages, not a mistake.

Because aquae is genitive singular, meaning of water.

So:

• ampulla = bottle
• aquae = of water

Together, ampulla aquae means a bottle of water.

This is a very common Latin pattern:

• vas vini = a vessel of wine
• copia aquae = a supply of water

English often uses of, while Latin often uses the genitive case.

They go together as one noun phrase: ampulla aquae = the bottle of water.

The main noun is ampulla, and aquae depends on it and tells you what kind of bottle it is.

So the two things that are standing are:

• ampulla aquae = a bottle of water
• vas parvum = a small vessel / small vase

Because the subject is compound: there are two things.

The two subjects are:

• ampulla aquae
• vas parvum

They are joined by et (and), so Latin uses a plural verb:

• stant = they stand / are standing

If there were only one item, you would expect stat.

Because parvum must agree with vas.

Vas is neuter singular, so the adjective also has to be neuter singular:

• vas = neuter singular
• parvum = neuter singular

Latin adjectives agree with the nouns they describe in:

• gender
• number
• case

So:

• feminine noun → maybe parva
• masculine noun → maybe parvus
• neuter noun → parvum

Here vas is neuter, so parvum is correct.

Vas means vessel, container, or sometimes vase, depending on context.

A learner may notice that it does not look like a first- or second-declension noun. That is because vas is a neuter noun with its own declensional pattern. In this sentence, you mainly need to know that it is:

• nominative singular
• neuter
• one of the two subjects of stant

That is why its adjective is parvum.

Because Latin word order is more flexible than English word order.

Both of these can be grammatical Latin:

• vas parvum
• parvum vas

Latin adjectives can come before or after the noun. Sometimes word order adds emphasis or style, but often either order is possible.

So vas parvum is perfectly normal.

Latin relies much more on case endings than on fixed word order.

In English, word order does a lot of grammatical work:

• The bottle and the vase stand on the table

In Latin, endings already show the relationships:

• mensa is ablative after in
• ampulla and vas are nominative subjects
• aquae is genitive
• stant is plural

Because the grammar is shown by endings, Latin can place words more freely. This sentence starts with in mensa to set the scene first: On the table ...

Because Classical Latin has no articles.

So ampulla can mean:

• a bottle
• the bottle

and vas parvum can mean:

• a small vase
• the small vase

You decide from context which English article is best.

Literally, stant means stand or are standing.

But in a sentence like this, it often means that the objects are standing there or are placed there upright. English may translate this more naturally as:

• are standing
• stand
• sometimes simply are on the table, depending on style

So the verb keeps a physical sense, but English may express it more loosely.

Because Latin often places the verb near the end of the sentence.

That is one of the most common Latin patterns, though not a strict rule. Here the sentence builds up the scene first:

• In mensa = location
• ampulla aquae et vas parvum = the things
• stant = what they are doing

So the final verb position is very natural Latin.

No, not naturally here.

The structure is:

• ampulla aquae = one noun phrase
• et = joins it to
• vas parvum = another noun phrase

So the sentence divides clearly into two coordinated subjects:

1. ampulla aquae
2. vas parvum

The genitive aquae belongs with ampulla, not with vas.