Si cives prudentes erunt, facile decernent quem magistratum creare velint.

It has three layers:

• Si cives prudentes erunt = the if-clause
• facile decernent = the main clause
• quem magistratum creare velint = an indirect question depending on decernent

So the main idea is:

• If the citizens are/will be wise, they will easily decide
• decide what?
• which magistrate they want to elect

Because the sentence is talking about a future situation.

• erunt = they will be
• decernent = they will decide

Latin often uses the future in both parts of a future condition:

• si ... erunt = if ... they will be / if they are
• ... decernent = ... they will decide

In English, we usually say If the citizens are wise, they will decide, even though Latin uses a future form in the if-clause.

Cives is nominative plural.

It is the subject of both:

• erunt
• decernent

So cives means the citizens, and they are the ones who:

• will be wise
• will decide

Prudentes agrees with cives.

Both are:

• nominative
• plural

Here prudentes is a predicate adjective, because it goes with erunt:

• cives prudentes erunt = the citizens will be wise

It is not describing a separate group like wise citizens in isolation; it is part of the statement about what the citizens will be.

Facile is an adverb, meaning easily.

It modifies decernent:

• facile decernent = they will decide easily

Even though facile can look like an adjective form, here its job is clearly adverbial because it describes how they will decide.

Quem magistratum means which magistrate.

Grammar:

• quem = accusative masculine singular
• magistratum = accusative masculine singular
• they agree with each other

This is an indirect question, so quem is the question word introducing the idea which magistrate.

Also, quem magistratum is the object of creare:

• quem magistratum creare = which magistrate to elect

Because quem magistratum creare velint is an indirect question.

In Latin, indirect questions normally use the subjunctive, so:

• direct question: Quem magistratum volunt creare?
• indirect question: decernent quem magistratum creare velint

So velint does not mean the sentence is doubtful in English. It is subjunctive mainly because Latin requires it in this construction.

Its tense is present subjunctive, which fits after a main verb in a primary tense such as the future decernent.

Creare is an infinitive because it depends on velint:

• velint creare = they want to elect

This is a normal complementary infinitive after a verb of wanting.

Also, in Roman political language, creare often means:

• to elect
• to appoint

So here it does not mean to create in the ordinary English sense.
Magistratum creare is a standard political expression for electing a magistrate.

The understood subject is the same cives.

So the sense is:

• the citizens will decide
• which magistrate
• they want to elect

Latin often leaves the subject unstated when it is already clear from the context and the verb ending.

Here:

• velint = they may wish / they want
• that they is the same group as cives

Latin word order is much freer than English word order.

A few things are happening here:

• quem comes first because question words commonly come early
• magistratum stays with quem
• creare comes before velint
• velint comes last, which is a very common place for the verb in a subordinate clause

So the order is natural Latin, even though English would usually say something like:

• which magistrate they want to elect

rather than

• which magistrate to elect they want

It is an interrogative word here, not a relative pronoun.

You can tell because the clause is an indirect question after decernent:

• they will decide which magistrate they want to elect

So quem means which or what, not whom in the sense of a relative clause like the man whom...

Formally, the shape quem can look the same in different uses, but here its function is interrogative.