Nescimus utrum hoc agmen ad castra redeat an ad urbem festinet.

The sentence has:

• a main clause: Nescimus = We do not know
• an indirect question: utrum hoc agmen ad castra redeat an ad urbem festinet = whether this column returns to camp or hastens to the city

So the whole sentence means:

• We do not know whether this column is returning to camp or hurrying to the city.

Utrum ... an is a standard Latin way to introduce a two-part indirect question:

• utrum = whether
• an = or

So:

• utrum ... redeat an ... festinet = whether it is returning ... or hurrying ...

This is very common when Latin sets up two alternatives.

Because classical Latin normally uses utrum, num, an, or another interrogative form for an indirect question, not si.

English often uses if and whether in similar ways:

• We do not know if/whether...

But Latin usually distinguishes them more clearly:

• si = if in a condition
• utrum ... an = whether ... or in a question

So here utrum ... an is the natural choice.

Because they are inside an indirect question after Nescimus.

A very important rule in Latin is:

• Indirect questions take the subjunctive.

So even though English says:

• We do not know whether it returns / is returning

Latin uses:

• redeat
• festinet

rather than indicative forms like redit or festinat.

Because the main verb is Nescimus, which is present tense. After a present main verb, Latin normally uses primary sequence, so the verbs in the indirect question are often in the present subjunctive when the action is contemporaneous.

So:

• Nescimus ... redeat ... festinet = We do not know whether it is returning ... or hurrying ...

If the main verb were past, you would usually expect imperfect subjunctives:

• Nesciebamus utrum ... rediret an ... festinaret = We did not know whether it was returning ... or hurrying ...

Hoc agmen is nominative singular, and it is the subject of both redeat and festinet.

Breaking it down:

• hoc = this
  
  • nominative singular neuter
• agmen = column, line of march, army on the move
  
  • nominative singular neuter

They agree because hoc modifies agmen.

So hoc agmen means:

• this column
• or more loosely, this marching force

Not exactly. Agmen does not simply mean army in the broadest sense. It usually suggests:

• a marching column
• a line of troops on the move
• a body of men advancing

So it has more of a movement or formation sense than exercitus, which is the ordinary word for army.

Depending on context, English might translate it as:

• column
• troop column
• marching force
• sometimes just army, if a smoother translation is needed

Because castra is one of those Latin nouns that is usually plural in form even when the meaning is singular in English.

So:

• castra = camp
• literally the form is plural, but English normally translates it as singular

This is often called a plural-only or pluralia tantum noun.

That is why Latin says:

• ad castra = to the camp

not a singular form like ad castrum.

Because they follow ad, and here ad shows motion toward a place.

So:

• ad castra = to/toward the camp
• ad urbem = to/toward the city

In Latin, ad + accusative is very common for motion toward something.

Because Latin, like English, can leave out repeated material when it is already understood.

So:

• utrum hoc agmen ad castra redeat an ad urbem festinet

really means:

• whether this column returns to camp or whether this column hastens to the city

But repeating hoc agmen would be unnecessary. Latin often avoids repeating the same subject when it clearly applies to both verbs.

They are not exact parallels in meaning:

• redeat = return, go back
• festinet = hasten, hurry

So the alternatives are slightly different in nuance:

• ad castra redeat = go back to camp
• ad urbem festinet = hurry to the city

The second verb suggests more urgency or speed. It is not just goes to the city, but makes haste toward the city.

They are:

• nescimus from nescio, nescire, nescivi/nescii, nescitum = not know
• redeat from redeo, redire, redii/redivi, reditum = go back, return
• festinet from festino, festinare, festinavi, festinatum = hasten, hurry

And the forms in the sentence are:

• nescimus = 1st person plural present indicative active
• redeat = 3rd person singular present subjunctive active
• festinet = 3rd person singular present subjunctive active

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

This sentence is arranged in a very natural Latin way:

• Nescimus first gives the main idea
• utrum ... an ... then lays out the two alternatives
• hoc agmen appears early as the subject of both alternatives
• each destination follows ad

English depends heavily on order, but Latin can move words around for:

• clarity
• emphasis
• rhythm
• style

So the Latin order is not strange; it is just less rigid than English.

A direct question would usually use the indicative, not the subjunctive:

• Utrum hoc agmen ad castra redit an ad urbem festinat?
• Is this column returning to camp or hurrying to the city?

Then when the question is reported indirectly after Nescimus, the verbs become subjunctive:

• Nescimus utrum hoc agmen ad castra redeat an ad urbem festinet.

That is a very useful comparison to remember.