Post longam disputationem cives constituunt ut porta urbis nocte firmius custodiatur.

Because post is a preposition that takes the accusative case. So:

• post = after
• longam disputationem = a long discussion / debate

That is why both longam and disputationem are accusative singular feminine.

Because longam agrees with disputationem.

• disputatio is feminine
• here it is singular
• and it is accusative because of post

So the adjective must match it in gender, number, and case:

• longa = nominative feminine singular
• longam = accusative feminine singular

Cives is the subject of constituunt.

• cives = the citizens
• constituunt = they decide

So cives constituunt means the citizens decide.

A learner may notice that cives can be either nominative plural or accusative plural in form. Here context shows it must be nominative plural, because it is the doer of constituunt.

Here constituunt means something like:

• they decide
• they resolve
• they determine

It does not mean they build or they establish in this sentence.

Latin verbs often have a range of meanings, and constituere can mean to decide when followed by an ut clause.

After verbs of deciding, ordering, arranging, or bringing something about, Latin often uses ut + subjunctive to express the content of the decision.

So:

• constituunt ut... = they decide that...
• or more naturally in English, they decide that ... should ...

This kind of ut clause is often called a substantive clause or an indirect command, depending on how a textbook classifies it.

Because it is inside the ut clause after constituunt.

Latin normally uses the subjunctive in this kind of clause:

• constituunt ut porta urbis... custodiatur
• the citizens decide that the city gate should be guarded...

So the subjunctive here does not mean doubt. It is required by the construction.

Because the sentence is focusing on the gate as the thing being guarded.

• custodiat would mean he/she/it guards
• custodiatur means he/she/it is guarded or should be guarded

Since porta is the subject of the ut clause, the passive makes sense:

• porta ... custodiatur = the gate ... should be guarded

If Latin used the active, it would need someone as the subject doing the guarding.

Custodiatur is:

• 3rd person singular
• present
• passive
• subjunctive

From the verb custodire = to guard.

It is singular because its subject is porta, which is singular.

Because the sentence is talking about one gate.

• porta = gate
• porta urbis = the gate of the city / the city gate

There is nothing strange about using the singular. The speaker simply has one particular gate in mind.

Because urbis depends on porta and shows possession or relationship:

• porta urbis = the gate of the city

This is a very common use of the genitive in Latin. English often uses either of or a noun used like an adjective:

• the gate of the city
• the city gate

Both reflect porta urbis.

Nocte is ablative singular, and here it is an ablative of time when:

• nocte = at night

Latin often uses the ablative without a preposition for expressions of time, especially with words like:

• nocte = at night
• die = by day / on a day
• aestate = in summer

So nocte by itself is completely normal.

Firmius is an adverb, not an adjective. It means:

• more firmly
• more securely
• more strongly

It modifies the verb custodiatur, not the noun porta.

So Latin is saying that the gate should be guarded more securely, not that the gate itself is firmer.

This is the comparative adverb form, which often ends in -ius.

Latin often leaves that unstated. Firmius can imply:

• more firmly than before
• more firmly than usual
• more firmly than is currently being done

English can do the same thing. A comparative does not always need an explicit second term.

Because Latin word order is much freer than English word order.

Putting the verb at the end is very common, especially in subordinate clauses. Here the order helps the sentence build toward the main action of the ut clause:

• ut porta urbis nocte firmius custodiatur

English depends much more on word order for grammar, but Latin uses endings more heavily, so it can move words around for emphasis, rhythm, or style.

Because Latin has no definite or indefinite articles.

So Latin does not have separate words for:

• the
• a
• an

Whether a noun should be understood as the gate, a gate, the citizens, or just citizens depends on context. In this sentence, English naturally uses the citizens and the city gate, but Latin does not need articles to express that.