Aqua in fonte purior est quam aqua in flumine.

They are both ablative singular.

Here, in means in / inside / in the case of location, and when in expresses location, it takes the ablative:

• in fonte = in the spring/fountain
• in flumine = in the river

So:

• fons → fonte in the ablative singular
• flumen → flumine in the ablative singular

A native English speaker often expects a preposition just to stay the same without affecting the noun, but in Latin the preposition usually determines the noun’s case.

Purior is the comparative form of purus, meaning pure or clean.

So:

• purus = pure, clean
• purior = purer, cleaner

The sentence is making a comparison:

• water in the spring is purer
• than water in the river

That is why Latin uses the comparative adjective purior.

Because it has to agree with aqua.

Aqua is:

• feminine
• singular
• nominative (it is the subject)

So the adjective describing it must also be:

• feminine
• singular
• nominative

The comparative purior is the correct nominative singular form for masculine/feminine.

So:

• aqua ... purior est = the water ... is purer

Quam means than in a comparison.

In this sentence:

• purior est quam aqua in flumine
• is purer than water in the river

A very common Latin pattern is:

• comparative adjective + quam
• for example, maior quam = bigger than
• melior quam = better than
• purior quam = purer than

Yes, Latin could leave it out.

The full sentence says:

• Aqua in fonte purior est quam aqua in flumine.

But Latin could also say:

• Aqua in fonte purior est quam in flumine.

The second aqua is understood from the first one.

So why repeat it? Usually for clarity, balance, or emphasis. English often does the same:

• Water in the spring is purer than water in the river rather than
• Water in the spring is purer than in the river

Both are possible, but the repeated noun can sound clearer.

Because the main subject of the sentence is only the first aqua.

The structure is:

• Aqua in fonte = subject
• purior est = is purer
• quam aqua in flumine = than water in the river

The second aqua is part of the comparison, not a second subject joined to the first. So the verb stays singular:

• est = is

Yes. With in + ablative, the meaning is normally in or on in the sense of location.

So:

• in fonte = in the spring/fountain
• in flumine = in the river

If Latin wanted to say from the spring, it would normally use a different preposition, such as:

• ex fonte = out of / from the spring

This is an important contrast:

• in + ablative = where?
• in + accusative = into where?

A literal layout is:

• Aqua = water
• in fonte = in the spring
• purior = purer
• est = is
• quam = than
• aqua = water
• in flumine = in the river

So, very literally:

• Water in the spring purer is than water in the river.

That sounds odd in English, but it is perfectly normal Latin.

Latin word order is more flexible than English because the noun endings show how the words function. The sentence could be rearranged in various ways without changing the basic meaning, though some orders sound more natural or more emphatic than others.

Yes, that is exactly how it works.

After quam, the thing being compared is stated:

• aqua in fonte = water in the spring
• aqua in flumine = water in the river

So the comparison is between water in the spring and water in the river.

This is simpler than some other Latin comparison patterns, because both sides are stated fully.

It can mean either, depending on context.

Since purus can mean pure, clean, or clear, purior can be translated in different natural English ways:

• purer
• cleaner
• sometimes even clearer, depending on context

If the meaning shown to the learner is something like Water in the spring is cleaner than water in the river, that is a natural English rendering of the Latin.

They are:

• aqua, aquae — water
• fons, fontis — spring, fountain, source
• flumen, fluminis — river
• purus, -a, -um — pure, clean
• sum, esse — to be
• quam — than

This helps explain the forms in the sentence:

• fonte comes from fons, fontis
• flumine comes from flumen, fluminis
• purior comes from the comparative of purus

It can be either, because fons has a range of meanings, including:

• spring
• fountain
• source

So in fonte could mean:

• in the spring
• in the fountain
• sometimes at the source

The best translation depends on context. If the sentence is contrasting natural water sources, spring is often the most natural choice.

Sometimes Latin does omit forms of to be, especially in poetry or very compressed style, but in ordinary prose est is very normal here.

So:

• Aqua in fonte purior est quam aqua in flumine is the straightforward full sentence.

Leaving out est would be less standard for a beginner’s model sentence.

In a common classroom-style restored pronunciation, roughly:

• AH-kwa in FON-teh POO-ree-or est kwahm AH-kwa in FLOO-mee-neh

A few helpful points:

• aqua: the qu sounds like kw
• c in classical Latin is always hard, but there is no c here except inside spelling like aqua, where it is part of qu
• purior has four syllables: pu-ri-or
• flumine has three syllables: flu-mi-ne

So the whole sentence has a steady, clear rhythm.