Quamvis res publica magna sit, singuli tamen cives bonum commune iuvare possunt.

Because quamvis meaning although or even though is normally followed by the subjunctive in Classical Latin.

So:

• res publica magna est = the state is great/large
• quamvis res publica magna sit = although the state is great/large

Here sit is the present subjunctive of esse. For an English speaker, this can feel strange, because English usually keeps the normal indicative after although. Latin often does not.

Here quamvis means although, even though, or however much.

It introduces a concessive clause: a clause that admits one fact, but then goes on to say something that seems surprising in spite of that fact.

So the structure is:

• quamvis ... sit = although ... is
• main clause: singuli tamen cives ... possunt = nevertheless individual citizens can ...

The idea is: Even though the state is large, individual citizens can still help the common good.

Because Latin often likes to pair a concessive word such as quamvis with tamen in the main clause.

A useful way to feel the structure is:

• quamvis = although
• tamen = nevertheless / still

So Latin is doing something very natural rhetorically:

• Although X, nevertheless Y

English can do this too, but we do not always use both words together. In Latin, this pairing is very common and clear.

Literally, res publica means the public thing or the public affair.

It became a standard Latin expression meaning things like:

• the state
• the commonwealth
• the republic

It is two words because it began as an ordinary noun phrase:

• res = thing, affair, matter
• publica = public

Over time, the phrase took on a more specific political meaning.

Because it agrees with res publica, which is grammatically feminine singular.

Latin adjectives must agree with the nouns they describe in:

• gender
• number
• case

Here:

• res publica = feminine singular nominative
• magna = feminine singular nominative

So magna matches res publica.

Singuli means individual, one by one, or each separately.

That is different from omnes, which means all.

So:

• omnes cives = all the citizens
• singuli cives = individual citizens, citizens one by one

The sentence is emphasizing the role of each person, not just the group as a whole. That is an important nuance.

Because cives is the subject of possunt.

The main clause is:

• singuli tamen cives ... possunt
  = individual citizens nevertheless can ...

Since cives is the subject, it must be in the nominative case. It is plural because the sentence is talking about more than one citizen.

Bonum commune is accusative singular neuter, and it means the common good.

Why accusative? Because it is the direct object of iuvare.

Breakdown:

• bonum = good here used as a noun, good thing / good
• commune = common, agreeing with bonum

Together they form a phrase meaning the common good.

This is a very common idea in political and moral writing.

Because possum means to be able, can.

So:

• iuvant = they help
• iuvare possunt = they can help

Latin often uses possum + infinitive the same way English uses can + verb.

Here:

• iuvare = present active infinitive, to help
• possunt = they are able

Together: they can help

Because it depends on possunt.

After verbs of being able, wanting, daring, beginning, and similar ideas, Latin often uses an infinitive. So:

• possunt iuvare = they can help
• literally, they are able to help

This is very close to English grammar.

Yes. Even though it is made of two words, the phrase functions as a single singular idea here: the state or the republic.

That is why the verb is singular:

• res publica ... sit
• not sint

Latin does this with some fixed expressions and compound ideas.

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

Latin uses endings, not position alone, to show what each word is doing. That means the writer can move words around for:

• emphasis
• rhythm
• style
• clarity

Putting possunt at the end is very normal Latin style. English usually prefers subject + verb + object, but Latin does not have to follow that pattern.

Possibly, yes. Magnus can mean not only large in size but also great, important, or powerful, depending on context.

So res publica magna sit could suggest:

• a large state
• a great state
• an important commonwealth

A learner should remember that Latin adjectives often have a wider range than a single English gloss.

Because Latin has no articles.

There is no separate word for the or a/an. Whether something should be translated as a state, the state, a common good, or the common good depends on context.

So:

• res publica can mean the state or a state
• bonum commune can mean the common good

English has to supply the article, but Latin does not.

A simple breakdown is:

• Quamvis = although
• res publica = the state / republic
• magna = great / large
• sit = may be / is in the subjunctive after quamvis
• singuli = individual
• tamen = nevertheless
• cives = citizens
• bonum commune = the common good
• iuvare = to help
• possunt = can

So the structure is:

• concessive clause: Although the state is great/large
• main clause: nevertheless individual citizens can help the common good

That is a very typical Latin sentence pattern.