Aqua in frigidario frigidissima est.

The structure is:

• Aqua = the subject, water
• in frigidario = a prepositional phrase, in the frigidarium / in the cold room
• frigidissima = a predicate adjective, describing the subject
• est = is

So the sentence is built like Subject + place phrase + adjective + is.

Because aqua is the subject of the sentence: it is the thing that is something.

Latin uses the nominative for the subject, so aqua is nominative singular. Its dictionary form is aqua, aquae, a first-declension feminine noun.

Because in takes different cases depending on meaning:

• in + ablative = in / on a place, showing location
• in + accusative = into / onto a place, showing motion toward

Here the meaning is location: the water is in the frigidarium, not moving into it. So Latin uses the ablative:

• frigidario = ablative singular

If the idea were into the frigidarium, then you would expect in frigidarium.

Frigidario is the ablative singular of frigidarium, a neuter second-declension noun.

A frigidarium was the cold room or cold bath area in a Roman bath complex. So in frigidario means in the cold room or in the frigidarium.

Because it agrees with aqua.

Latin adjectives must agree with the nouns they describe in:

• gender
• number
• case

Since aqua is feminine singular nominative, the adjective must also be feminine singular nominative. That is why you get frigidissima, not frigidissimus or frigidissimum.

It is the superlative form of frigidus, frigida, frigidum, meaning cold.

The degrees are:

• frigidus = cold
• frigidior = colder
• frigidissimus / frigidissima / frigidissimum = coldest, or sometimes very cold

Here frigidissima is feminine nominative singular to match aqua.

Grammatically, it is a superlative, so its most basic sense is coldest.

However, Latin superlatives can also be used in an intensive sense, meaning something like very cold or extremely cold, especially when no explicit comparison is stated.

So depending on context, frigidissima est may be understood as:

• is the coldest
• or is very cold / extremely cold

Because frigidissima is linked to aqua by est. It is a predicate adjective, and predicate adjectives normally stand in the same case as the subject, which here is the nominative.

So:

• aqua = nominative subject
• frigidissima = nominative adjective describing that subject

The phrase in frigidario is separate; it just tells you where.

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

Placing est at the end is very common in Latin, especially in straightforward statements. It can sound natural and orderly in Latin even though English usually prefers the verb earlier.

So Aqua in frigidario frigidissima est is perfectly normal Latin word order.

Yes. Because the endings carry the grammar, Latin can rearrange the words without changing the basic meaning.

For example, these would still mean essentially the same thing:

• In frigidario aqua frigidissima est
• Frigidissima aqua in frigidario est
• Aqua frigidissima in frigidario est

The difference is mainly one of emphasis or style, not basic grammar.

Because Classical Latin has no articles.

So aqua can mean:

• water
• the water
• sometimes even some water

And frigidario can mean:

• in the frigidarium
• in a frigidarium

You figure out the most natural English wording from the context.

Yes, potentially, because a superlative naturally suggests comparison. But Latin does not always have to state the comparison explicitly.

A sentence with frigidissima can simply leave the comparison understood from context, or use the superlative in the looser sense very cold.

If Latin wanted to make the comparison clearer, it could add something like:

• omnium = of all
• or some other phrase showing the group being compared

But this sentence works on its own without that extra information.