Avia dicit fluvium in mare fluere.

Because this sentence uses indirect statement, a very common Latin construction.

After a verb like dicit (says), Latin often expresses that ... by using:

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

So in:

Avia dicit fluvium in mare fluere.

• avia = the subject of dicit
• fluvium = the subject of fluere, but in the accusative
• fluere = the infinitive verb

In English we say Grandmother says that the river flows into the sea, but Latin structures it differently.

It is the accusative-and-infinitive construction, often called indirect statement.

Pattern:

• main verb of saying/thinking/perceiving
• accusative subject
• infinitive verb

Here that gives:

• dicit = says
• fluvium = the river
• fluere = to flow

So literally it is something like:

Grandmother says the river to flow into the sea

but in normal English:

Grandmother says that the river flows into the sea.

Because in Latin indirect statement, the verb of the reported idea goes into the infinitive.

Compare:

• Fluvius in mare fluit. = The river flows into the sea.
  This is a direct statement.

• Avia dicit fluvium in mare fluere. = Grandmother says that the river flows into the sea.
  This is an indirect statement.

So fluere is not just a dictionary form here by accident; it is required by the construction after dicit.

Because Latin usually does not use a separate word corresponding to English that in this kind of sentence.

English says:

Grandmother says that the river flows into the sea.

Latin typically says:

Grandmother says the-river to-flow into the sea.

So the idea of English that is built into the accusative-and-infinitive construction itself.

Yes, in can take either the accusative or the ablative, depending on the meaning.

• in + accusative = into, showing motion toward/into something
• in + ablative = in/on, showing location

Here the river is flowing into the sea, so Latin uses:

• in mare

If it were describing location instead, you would expect in with the ablative.

Because mare is a third-declension neuter noun.

Its basic forms are:

• nominative singular: mare
• accusative singular: mare
• genitive singular: maris

For neuter nouns, the nominative and accusative are often the same. So mare can be either nominative or accusative depending on its role in the sentence.

Here it is accusative because it follows in in the sense into the sea.

Avia is nominative singular, and it is the subject of dicit.

So:

• avia = grandmother
• dicit = says

Together:

• Avia dicit = Grandmother says

Then the rest of the sentence, fluvium in mare fluere, is what grandmother says.

Not in this sentence.

At first glance, a learner might think:

• dicit = says
• fluvium = river in the accusative
  so maybe river is the object of says

But say does not normally take a noun like river as its direct object in this meaning. Also, the presence of the infinitive fluere strongly signals indirect statement.

So fluvium is not the object of dicit. It is the subject of the infinitive fluere, put into the accusative because of the construction.

Very literally, it is:

Grandmother says the river into the sea to flow.

That is not natural English, but it helps show the Latin structure:

• Avia = grandmother
• dicit = says
• fluvium = river
• in mare = into the sea
• fluere = to flow

Natural English then becomes:

Grandmother says that the river flows into the sea.

Because Latin has no articles.

So:

• avia can mean grandmother, a grandmother, or the grandmother
• fluvium can mean river, a river, or the river
• mare can mean sea, a sea, or the sea

English has to choose one, but Latin leaves that to context. In many teaching sentences, English may supply the because it sounds most natural.

Yes. Latin word order is much freer than English word order because the endings show the grammatical roles.

So these could express the same basic meaning:

• Avia dicit fluvium in mare fluere.
• Avia fluvium in mare fluere dicit.
• Fluvium in mare fluere avia dicit.

The original order is perfectly normal, but Latin often moves words around for emphasis, rhythm, or style.

Dicit is present tense, third person singular, from dicere.

Depending on context, English might translate it as:

• says
• is saying
• sometimes even does say, for emphasis

In a simple sentence like this, says is the most straightforward translation.

The direct statement would be:

Fluvius in mare fluit.
The river flows into the sea.

Then, if you want to report that statement after dicit, Latin changes it into indirect statement:

• fluvius becomes fluvium
• fluit becomes fluere

So:

• direct: Fluvius in mare fluit.
• indirect after dicit: Avia dicit fluvium in mare fluere.

That change is one of the most important patterns in Latin syntax.