Pater dicit fluvium hodie tardius fluere, quia nox frigida fuit.

Yes, the river is the subject of fluere, but in Latin, after a verb of saying like dicit, an indirect statement is usually built with:

• an accusative noun as the subject of the reported statement
• an infinitive as the verb of the reported statement

So:

• Pater dicit = Father says
• fluvium ... fluere = that the river ... is flowing

Even though fluvium is logically the subject of fluere, it appears in the accusative because of this construction.

If the statement were direct instead, you would get:

• Fluvius hodie tardius fluit. = The river is flowing more slowly today.

There, fluvius is nominative because it is the subject of a normal finite verb.

Because Latin often uses the accusative-and-infinitive construction for indirect statement.

After dicit, Latin does not usually say the equivalent of that the river flows with a separate word for that and a finite verb. Instead, it says:

• fluvium ... fluere

literally something like:

• the river ... to flow

But in normal English, we translate this as:

• that the river is flowing
• that the river flows

So fluere is not just any infinitive; it is the standard infinitive used inside indirect statement after verbs like:

• dicit = says
• putat = thinks
• audit = hears
• videt = sees

Because classical Latin usually does not need one in this kind of sentence.

English says:

• Father says that the river is flowing more slowly today.

Latin normally says:

• Pater dicit fluvium hodie tardius fluere.

The idea of that is built into the accusative-plus-infinitive structure itself. So instead of using a separate conjunction, Latin signals reported speech by grammar.

This is one of the biggest differences between English and Latin syntax.

Tardius is a comparative adverb meaning more slowly.

It comes from tardus, -a, -um = slow.
The adverb system works like this:

• tardē = slowly
• tardius = more slowly
• tardissimē = most slowly

So fluvium hodie tardius fluere means:

• the river is flowing more slowly today

The comparison may be with:

• its usual speed
• how it flowed before
• how it flows at other times

Latin does not always state the second half of the comparison explicitly, just as English often doesn’t.

Fuit is the perfect tense of sum, while erat is the imperfect.

Here, nox frigida fuit means:

• the night was cold
• more literally, the night has been cold / was cold as a completed fact

The perfect tense presents the cold night as a completed event in the past. That fits well here, because the cold night happened before the father’s statement about the river today.

If Latin used erat, that would give more of a background or ongoing-description feel:

• the night was being cold / was cold in a descriptive sense

Both can sometimes be translated simply as was, but the nuance is different:

• fuit = completed past fact
• erat = ongoing/background past situation

Because quia here introduces a reason presented as a real fact:

• quia nox frigida fuit = because the night was cold

The verb fuit is therefore in the indicative, the normal mood for factual statements.

For a beginner, the safest rule is:

• quia
  • indicative = because introducing an ordinary factual reason

Later, you may see more complicated uses in some authors, but here the indicative is exactly what you should expect.

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

This sentence is arranged very naturally:

• Pater = subject first
• dicit = main verb early, introducing the indirect statement
• fluvium hodie tardius fluere = the content of what he says
• quia nox frigida fuit = the reason, placed afterward

A very literal breakdown is:

• Father says
• the river
• today
• more slowly
• to be flowing
• because the night was cold

Latin could rearrange some of these words without changing the basic meaning, but the chosen order is clear and smooth.

Because it agrees with nox.

Latin adjectives must agree with the nouns they describe in:

• gender
• number
• case

Here:

• nox is feminine singular nominative
• so frigida must also be feminine singular nominative

That is why we get:

• nox frigida = a cold night / the night was cold

Even though nox is a third-declension noun, the adjective frigida is first/second declension. That does not matter: adjectives and nouns do not have to belong to the same declension, only to agree in gender, number, and case.

Because Latin has no articles in the same way English does.

So:

• pater can mean father, a father, or the father
• fluvium can mean a river or the river
• nox can mean a night or the night

Context tells you which English article makes the most sense.

That means when translating from Latin to English, you often have to supply a, an, or the yourself.

A direct version would be:

• Fluvius hodie tardius fluit, quia nox frigida fuit.

That means:

• The river is flowing more slowly today because the night was cold.

Then, when it becomes indirect after dicit, Latin changes:

• fluvius → fluvium
• fluit → fluere

So:

• Fluvius ... fluit
  becomes
• fluvium ... fluere

This is a very useful way to understand indirect statement in Latin: imagine the direct sentence first, then convert its subject to the accusative and its verb to an infinitive.