Cum pluvia gravis cadit, cives per portam in urbem intrant.

Yes—cum has more than one use.

In this sentence, cum is a conjunction, introducing a clause:

• Cum pluvia gravis cadit = When heavy rain falls

But cum can also be a preposition meaning with, followed by the ablative:

• cum amico = with a friend

So the function depends on what follows:

• if cum introduces a verb and a whole clause, it often means when, since, or although
• if cum is followed by a noun in the ablative, it usually means with

Because they have different subjects.

• pluvia = rain → singular
  so cadit = falls
• cives = citizens → plural
  so intrant = enter

So:

• pluvia gravis cadit = heavy rain falls
• cives ... intrant = the citizens enter

Latin verbs change form to match their subjects in number (singular or plural).

Because gravis must agree with pluvia.

pluvia is:

• feminine
• singular
• nominative (the subject)

So the adjective gravis must also be:

• feminine
• singular
• nominative

That is why we get:

• pluvia gravis = heavy rain

This is a basic Latin rule: adjectives agree with the nouns they describe in gender, number, and case.

Because cives is the subject of intrant.

• cives = citizens
• intrant = enter

The people doing the action are the citizens, so cives is in the nominative case, which is the usual case for the subject of a sentence.

Its dictionary form is civis (citizen), and cives here is the nominative plural.

Because both are objects of prepositions that express motion.

• per portam = through the gate
• in urbem = into the city

Both per and in (when it means into) take the accusative.

So:

• porta becomes portam
• urbs becomes urbem

This is very common in Latin:

• per + accusative = through
• in + accusative = into, onto
• in + ablative = in, on (location, not motion)

They express two different ideas of movement.

• per portam = through the gate
  This tells you the route or path.
• in urbem = into the city
  This tells you the destination.

So the sentence is not repeating the same idea twice. It means something like:

• the citizens go through the gate
• and thereby move into the city

Because in changes meaning depending on the case.

• in + accusative = into (motion toward)
• in + ablative = in or inside (location)

So:

• in urbem intrant = they enter into the city
• in urbe sunt = they are in the city

Here there is movement into the city, so the accusative urbem is correct.

Latin word order is more flexible than English word order because the endings show each word’s role.

This sentence begins with the cum clause:

• Cum pluvia gravis cadit = When heavy rain falls

Then comes the main clause:

• cives per portam in urbem intrant

Starting with the time clause is very natural in Latin, just as in English:

• When heavy rain falls, the citizens enter the city through the gate.

Could the order be changed? Yes, in many ways:

• Cives cum pluvia gravis cadit per portam in urbem intrant
• Cives in urbem per portam intrant

But the given order is clear and natural. Latin often uses word order for emphasis or style, not just grammar.

Because Classical Latin has no articles.

So:

• pluvia can mean rain, the rain, or a rain
• cives can mean citizens or the citizens
• urbem can mean a city or the city

English has to choose an article when translating, but Latin usually leaves that to the context.

Because this is a straightforward temporal clause: when heavy rain falls.

The verb in the cum clause is:

• cadit = present indicative

With cum, Latin can use different moods depending on meaning:

• indicative for a simple time sense: when
• subjunctive in many clauses meaning since, although, or in more literary narrative uses

Here the sentence is simple and factual, so the indicative is expected.

Here they are:

• cum = when / with
• pluvia, pluviae = rain
• gravis, grave = heavy, serious
• cado, cadere, cecidi = fall
• civis, civis = citizen
• per = through
• porta, portae = gate
• in = into / in
• urbs, urbis = city
• intro, intrare = enter

And the forms used here are:

• pluvia = nominative singular
• gravis = nominative singular agreeing with pluvia
• cadit = 3rd person singular present
• cives = nominative plural
• portam = accusative singular
• urbem = accusative singular
• intrant = 3rd person plural present

Yes. gravis has a range of meanings depending on context, including:

• heavy
• serious
• weighty
• sometimes severe

With pluvia, the natural meaning is heavy:

• pluvia gravis = heavy rain

So context helps decide which English word is best.