Aqua deorsum e monte fluit.

Aqua is the subject of the sentence, so it is in the nominative singular.

• Dictionary form: aqua, aquae = water
• Nominative singular: aqua

In this sentence, aqua is the thing doing the action of fluit (flows), so nominative is the expected case.

Because after the preposition e (out of / from), Latin normally uses the ablative case.

The noun is:

• mons = mountain in the nominative
• monte = from the mountain / on the mountain / by the mountain depending on context, in the ablative singular

So:

• e monte = from the mountain

That is why you see monte, not mons.

Deorsum is an adverb, and it means downward, down, or downwards.

It tells you how the water flows:

• fluit = flows
• deorsum fluit = flows downward

So the sentence has both:

• deorsum = the direction downward
• e monte = the starting point from the mountain

Together they give the idea of water flowing down from the mountain.

They are related, but they do different jobs.

• deorsum tells the direction of movement: downward
• e monte tells the place it comes from: from the mountain

So the sentence is not just saying that the water comes from the mountain. It is also emphasizing that it moves downward.

English often uses both kinds of information too:

• The water flows down from the mountain.

E and ex are just two forms of the same preposition, both meaning out of or from.

A common pattern is:

• e before a consonant
• ex before a vowel or h

Since monte begins with m, a consonant, e monte is the expected form.

You may still see some variation in Latin authors, but e monte is perfectly normal.

Fluit is:

• 3rd person singular
• present tense
• active voice
• from the verb fluo, fluere = to flow

So fluit means:

• he/she/it flows

Here the subject is aqua, which is singular, so fluit matches it:

• aqua fluit = water flows

Latin often places the verb near the end of the sentence, although Latin word order is much freer than English word order.

So:

• Aqua deorsum e monte fluit

is a very natural Latin arrangement.

But Latin could also rearrange the words for emphasis, for example:

• E monte aqua deorsum fluit
• Deorsum aqua e monte fluit

The basic meaning would stay the same because the case endings show the grammatical roles more clearly than word order does.

Classical Latin has no articles like English the or a/an.

So aqua can mean:

• water
• the water
• sometimes even a body of water, depending on context

Likewise e monte can mean:

• from the mountain
• from a mountain

The exact choice in English depends on context and natural translation.

A simple classroom-style pronunciation would be roughly:

• AH-kwa day-OR-soom ay MON-tay FLOO-it

A few helpful notes:

• aqua: qu is pronounced like kw
• deorsum: the eo is pronounced as two vowels, not like English ee
• e: pronounced like eh or ay, depending on the system being taught
• fluit: often pronounced roughly FLOO-it

If you are learning a specific pronunciation system, such as Classical or Ecclesiastical, your teacher may give slightly different values for some vowels.

Yes, de monte is possible Latin, but it is not exactly identical in feel.

Very broadly:

• e/ex often means out of / from within / from
• de often means down from / off / concerning, depending on context

With a mountain, both can make sense in some contexts, but e monte is a straightforward way to express from the mountain as the source.

In this sentence, deorsum already gives the idea of downward, so e monte works neatly for the source: from the mountain.