Flumen in mare fluit.

Flumen is nominative singular, which is the case normally used for the subject of a sentence.

We know it is the subject because:

• the nominative case commonly marks the doer or main thing being described
• the verb fluit is third person singular, so it matches a singular subject
• the meaning fits naturally: the river is what is flowing

So flumen = the river as the subject.

Flumen, fluminis is a third-declension neuter noun meaning river.

A learner may notice that it does not look like a first- or second-declension noun. That is because it belongs to the third declension, where noun forms are more varied. Its nominative singular is flumen, and its stem appears in forms like fluminis.

Latin often uses the preposition in to show movement into something.

So:

• in
  • accusative = into, onto, showing motion toward
• in
  • ablative = in, on, showing position or rest

Here the river is moving toward and into the sea, so Latin uses in mare = into the sea.

Because in here expresses motion into, it takes the accusative case. The accusative singular of mare is mare.

That may look confusing at first, because mare is a neuter third-declension noun, and many neuter nouns have the same form in the nominative and accusative singular.

So in this sentence:

• mare is accusative singular
• even though it looks the same as the nominative

Mare, maris means sea and is a third-declension neuter noun.

Like many neuter nouns:

• nominative singular: mare
• accusative singular: mare

That is why the form does not change visibly here, even though its grammatical role is different from flumen.

Fluit is the verb flows. It comes from fluo, fluere, meaning to flow.

In this sentence it is:

• present tense
• indicative mood
• active voice
• third person singular

So fluit means he/she/it flows, and here it means it flows, referring to flumen.

The ending -it is a common sign of third person singular present active for many Latin verbs of the third conjugation and some others.

Here:

• fluo = I flow
• fluit = he/she/it flows

Since flumen is singular, the singular verb form is used.

Yes. Latin word order is much freer than English word order because the cases show the grammatical relationships.

So all of these could express roughly the same idea:

• Flumen in mare fluit
• In mare flumen fluit
• Fluit flumen in mare

However, word order can affect emphasis or style. The given order is straightforward and natural:

• Flumen introduces the subject
• in mare shows where it is going
• fluit finishes with the verb

Latin often places the verb at or near the end, though this is not a strict rule.

Yes. The meaning depends on the case of the noun after it.

• in
  • ablative = in / on with no movement
    
    • example: in mari = in the sea
• in
  • accusative = into / onto with motion
    
    • example: in mare = into the sea

So in this sentence, because the river is moving toward the sea, in means into.

Because mari would be a different case.

With in:

• in mari would usually mean in the sea and use the ablative
• in mare means into the sea and uses the accusative

Since this sentence describes motion, Latin chooses mare, not mari.

No. Latin usually does not need an explicit subject pronoun, because the verb ending already tells you the person and number.

For example:

• fluit already means he/she/it flows

If the noun flumen is present, that clearly tells you what is flowing. A separate pronoun is unnecessary unless the speaker wants special emphasis.

Not especially in this sentence, but a learner may notice one useful pattern: many neuter nouns have the same form for nominative and accusative in the singular.

That is true here:

• flumen is nominative singular because it is the subject
• mare is accusative singular because it follows in expressing motion

Even though both nouns are neuter, their jobs in the sentence are different. Their roles are shown by syntax and the preposition, not just by visibly different endings.

Yes, depending on context. Since Latin has no articles, flumen could be the river or a river, and mare could be the sea or a sea.

In many textbook sentences, English uses the because it sounds natural and generic: The river flows into the sea. But A river flows into the sea is also grammatically possible as a translation if the context supports it.