Aqua in piscina frigidior est quam aqua in caldario.

Frigidior is the comparative form of frigidus, frigida, frigidum (cold).

So:

• frigida = cold
• frigidior = colder

Since the sentence compares the water in one place with the water in another place, Latin uses the comparative: frigidior.

Also note that frigidior here agrees with aqua, which is feminine singular, but in the comparative the same form frigidior is used for both masculine and feminine nominative singular.

Quam means than after a comparative.

So the pattern is:

• frigidior ... quam ... = colder ... than ...

In this sentence:

• Aqua in piscina frigidior est quam aqua in caldario.
• The water in the pool is colder than the water in the hot bath.

This is one of the most common ways Latin makes a comparison.

After in, Latin can use different cases depending on the meaning.

Here it means in in the sense of in/inside a place, not movement into a place. That takes the ablative.

So:

• in piscina = in the pool
• in caldario = in the hot bath / hot room

If there were motion into a place, Latin would usually use in with the accusative instead.

You can tell from their endings and from the preposition in.

• piscina here is the ablative singular of a first-declension noun.
• caldario here is the ablative singular of a second-declension neuter noun, caldarium.

Because both follow in with a location meaning, the ablative makes sense.

So the grammar is:

• in + ablative = location
• in piscina = in the pool
• in caldario = in the hot bath

Yes, Latin could leave it out if the meaning were clear.

You may see:

• Aqua in piscina frigidior est quam in caldario.

That would still mean The water in the pool is colder than in the hot bath, with aqua understood.

But repeating aqua makes the comparison clearer and more balanced:

• aqua ... quam aqua ...

Latin often repeats a word when it wants to be explicit or stylistically neat.

Only one est is needed because the second one is understood.

Latin often omits words that are obvious from the context. So:

• Aqua in piscina frigidior est quam aqua in caldario
  really means
• Aqua in piscina frigidior est quam aqua in caldario est

But repeating est would usually sound unnecessary.

This is similar to English:

• The water in the pool is colder than the water in the bath
  where we do not need to repeat is.

The basic pieces are:

• Aqua in piscina = subject phrase
• frigidior est = predicate
• quam aqua in caldario = comparison phrase

Latin word order is more flexible than English because endings show grammatical relationships. So this sentence could be rearranged in different ways without changing the basic meaning.

For example, Latin might also say:

• Aqua frigidior est in piscina quam in caldario
• In piscina aqua frigidior est quam aqua in caldario

But the original order is straightforward and easy to follow.

Because it describes aqua, which is singular.

The subject is:

• aqua = water

Even though English sometimes treats water as a mass noun, Latin still uses a singular noun here, so the adjective and verb are singular too:

• aqua — singular
• frigidior — singular
• est — singular

Aqua is a first-declension feminine noun.

Its basic form is:

• aqua, aquae = water

In this sentence, aqua is in the nominative singular because it is the subject of est.

Caldarium is a second-declension neuter noun.

It originally refers to the hot room or hot bath in a Roman bathing complex.

Its forms include:

• nominative singular: caldarium
• ablative singular: caldario

So in in caldario, the form is ablative singular because it follows in with the meaning of location.

Usually yes in a simple sentence like this, but the Latin word can have a somewhat broader range depending on context.

Piscina can refer to:

• a pool
• a fishpond
• a basin or reservoir

In many beginner sentences, pool is the most natural translation, but the exact sense depends on the situation being described.

Yes. Latin has another common way to make comparisons: the comparative adjective can be followed directly by the ablative of comparison instead of quam.

So a sentence like this could also be expressed as:

• Aqua in piscina frigidior est aqua in caldario

with the second noun phrase put into the ablative where appropriate.

However, for learners, the quam construction is often easier to recognize because it matches English than more closely.

In restored classical pronunciation, it is roughly:

• fri-gi-di-or

with the g always hard, as in get, not soft as in giant.

A rough guide:

• fri as in free but shorter
• gi with hard g
• di
• or

The important thing for a learner is to avoid turning the g into a j sound.

A very useful pattern is:

• [subject] + [comparative adjective] + est + quam + [thing compared]

In this sentence:

• Aqua in piscina = subject
• frigidior = comparative adjective
• est = is
• quam aqua in caldario = than the water in the hot bath

So the sentence is a good example of how Latin makes comparisons with a comparative adjective plus quam.