Pauci cives ad contionem veniunt, quia pluvia gravis cadit.

They are nominative plural, because they are the subject of veniunt (come).

• cives = nominative plural of civis (citizen)
• pauci = nominative plural masculine of paucus (few)

Together they mean few citizens, and they are the ones doing the action of coming.

Because cives is plural, so the adjective must also be plural.

Latin adjectives agree with the nouns they describe in:

• number
• gender
• case

So:

• paucus civis would mean one few citizen, which does not make sense
• pauci cives = few citizens

Because many third-declension nouns have nominative plural in -es.
The dictionary form is civis, civis.

Its nominative plural is:

• cives = citizens

So the ending -es here does not mean first declension or anything unusual; it is a normal third-declension plural ending.

Because it comes after ad, and ad takes the accusative when it means to or toward a place or event.

So:

• ad contionem = to the assembly / to the public meeting

The basic idea is motion toward something.

Contionem is the accusative singular of contio.

A contio is a public gathering, assembly, or meeting, especially one where people are addressed. In Roman contexts, it often means a public meeting of citizens.

So in this sentence, ad contionem veniunt means they come to the assembly/public meeting.

Because the subject is plural: pauci cives = few citizens.

So the verb must also be plural:

• venit = he/she/it comes
• veniunt = they come

Latin verbs change form to match the subject.

Both are present indicative active.

• veniunt = they come / are coming
• cadit = it falls / is falling

The present tense here describes what is happening now, or what is currently the case: Few citizens are coming to the assembly because heavy rain is falling.

Because Latin can describe rain as something that falls, just as English does.

So:

• pluvia gravis cadit = literally heavy rain falls

This is natural Latin. You may also learn pluit (it is raining), but here the sentence wants an actual noun, pluvia (rain), modified by gravis (heavy).

Because pluvia gravis is the subject of cadit.

So both words are nominative singular:

• pluvia = nominative singular
• gravis = nominative singular, agreeing with pluvia

If it were pluviam gravem, that would be accusative, which would not fit here because the rain is not a direct object; it is the thing doing the falling.

Latin word order is much more flexible than English word order.

Both of these are possible:

• pluvia gravis
• gravis pluvia

They both mean heavy rain. The sentence puts gravis after the noun, which is very normal in Latin. The choice often depends on style or emphasis, not basic grammar.

Quia means because and introduces a clause giving the reason.

So:

• quia pluvia gravis cadit = because heavy rain is falling

This clause explains why few citizens come to the assembly.

Latin often uses quia for straightforward factual reasons like this.

Because the sentence presents the reason as a fact.

• cadit is indicative
• the speaker is saying that the heavy rain really is falling

After quia, Latin often uses the indicative when the cause is presented as real and certain.

There is no separate word, because Latin has no articles.

So:

• cives can mean citizens, the citizens, or sometimes some citizens, depending on context
• contionem can mean an assembly or the assembly
• pluvia gravis can mean heavy rain or the heavy rain, depending on context

English must choose an article, but Latin usually leaves that to context.

Yes. Latin word order is flexible because the endings show the grammar.

For example, these could all mean basically the same thing:

• Pauci cives ad contionem veniunt
• Pauci cives veniunt ad contionem
• Ad contionem pauci cives veniunt

The meaning stays clear because:

• cives is the subject
• contionem is the object of ad
• veniunt is the verb

Changing the order usually changes emphasis, not the core meaning.

It can convey both ideas.

• pauci cives = few citizens
• in smoother English, that can also be not many citizens

So pauci often emphasizes small number rather than just counting precisely.