Post meridiem alterum agmen e monte descendit et ad flumen procedit.

Because agmen is a neuter noun.

In Latin, neuter nouns and adjectives often have the same form in the nominative and accusative. So:

• agmen can be nominative singular or accusative singular
• alterum can also be nominative singular neuter or accusative singular neuter

Here, alterum agmen is the subject, so it is nominative singular: the other/second column.

This is a very common beginner difficulty, because English speakers often first learn -um as a masculine accusative ending. But with neuter words, -um can perfectly well be nominative too.

Alter basically means the other of two.

So depending on context, alterum agmen can mean:

• the other column if there are two columns already in view
• the second column if English sounds more natural that way

Latin alter is not the ordinary word for another one in a general sense. It usually implies a contrast between one of two and the other.

So the exact English wording depends on the larger context, but grammatically alterum points to the other of two.

Agmen does not just mean army in the broadest possible sense. More specifically, it usually refers to a moving body of troops, a column, or a line of march.

So in a military narrative, agmen often suggests:

• troops on the move
• a marching formation
• a column of soldiers

That is why English often translates it as column, line, or sometimes force, depending on context.

Because post is a preposition that takes the accusative case when it means after.

So:

• post = after
• meridiem = accusative singular of meridies = midday/noon

Therefore post meridiem literally means after midday or after noon.

This is also the same Latin phrase behind the English abbreviation p.m.

Because e/ex means out of or from, and it takes the ablative case.

So:

• e = from / out of
• monte = ablative singular of mons = mountain

Together, e monte means from the mountain.

A beginner may wonder whether e or ex is correct. Both are forms of the same preposition:

• e is often used before consonants
• ex is often used before vowels, but usage can vary

So e monte is completely normal.

It is not bad repetition. The two parts do slightly different jobs:

• descendit tells you the kind of movement: goes down / descends
• e monte tells you the starting place: from the mountain

English does the same thing:

• he comes down from the mountain

So Latin is being perfectly natural here. The verb gives the action; the prepositional phrase gives the place the movement starts from.

Because ad takes the accusative case.

So:

• ad = to / toward
• flumen = accusative singular of flumen

Therefore ad flumen means to the river or toward the river.

This is a very common Latin pattern:

• motion toward a place → often ad + accusative
• motion from a place → often ab/de/e + ablative

So the sentence nicely shows both directions:

• e monte = from the mountain
• ad flumen = to the river

Formally, both are present tense, third person singular:

• descendit = he/she/it descends
• procedit = he/she/it advances/proceeds

In Latin narrative, the present tense is often used as a historical present. That means Latin uses the present to describe past events more vividly, as if they are happening before your eyes.

So depending on context, English may translate them as either:

• descends ... proceeds
  or
• descended ... proceeded

Both verbs are singular because the subject, alterum agmen, is singular.

Because Latin, like English, does not need to repeat the subject when it stays the same.

So:

• alterum agmen ... descendit et procedit

means that the same subject, alterum agmen, does both actions:

• it descends
• and it proceeds

Latin often avoids unnecessary repetition, especially when one subject governs two or more verbs joined by et.

Yes, but not in the same rigid way as in English.

Latin word order is more flexible because the case endings show how words relate to each other. Here the order is quite natural:

• Post meridiem sets the time first
• alterum agmen introduces the subject
• e monte descendit gives the first action
• et ad flumen procedit adds the second action

Putting Post meridiem first gives the time setting prominence: After midday...

So the order is not random, but it is more about emphasis and flow than about basic grammar.