Si fur innocens esset, iudex eum in carcerem non mitteret.

Because this sentence is a present contrary-to-fact conditional.

Latin often uses:

• imperfect subjunctive in the if-clause
• imperfect subjunctive in the main clause

to express something that is not actually true now, but is being imagined.

So:

• Si fur innocens esset = If the thief were innocent
• iudex eum in carcerem non mitteret = the judge would not send him to prison

The idea is: he is not innocent, so this is hypothetical and unreal.

Because erat would usually state a fact or a straightforward past situation, while esset marks the clause as hypothetical/unreal.

Compare:

• Si fur innocens erat... = If the thief was innocent...
  This sounds like a real possibility or a past factual condition.

• Si fur innocens esset... = If the thief were innocent...
  This signals an unreal or imagined situation.

So esset is chosen because Latin is showing that the condition is not true in reality.

In this type of conditional, Latin normally matches the verb forms:

• if-clause: imperfect subjunctive
• result clause: imperfect subjunctive

So:

• esset = were
• mitteret = would send

This pairing is standard for a present contrary-to-fact condition.

If Latin wanted to express a past contrary-to-fact idea, it would usually use the pluperfect subjunctive instead.

Fur is nominative singular because it is the subject of esset.

The clause is:

• fur = the thief
• innocens = innocent
• esset = were

So fur is the person being described as innocent.

Because innocens is a third-declension adjective with the same nominative singular form for masculine and feminine.

So:

• masculine: fur innocens = an innocent thief
• feminine: a feminine noun could also use innocens

It does agree with fur in case, number, and gender, but the visible form happens to stay innocens.

Eum is accusative singular masculine.

It is the direct object of mitteret, and it refers back to fur.

So the sentence literally has:

• iudex = the judge
• eum = him
• non mitteret = would not send

Latin often uses a pronoun like this even where English might simply repeat the noun less often.

Because once fur has already been mentioned, eum is a natural way to refer back to him.

Latin does this much like English uses him instead of repeating the thief.

So the structure is:

• first mention: fur
• later reference: eum

This avoids repetition and makes the sentence flow naturally.

Because in with the accusative usually shows movement toward something, while in with the ablative usually shows location in something.

So:

• in carcerem = into prison / to prison
• in carcere = in prison (location)

Since mitteret means would send, the idea is movement into prison, so Latin uses carcerem.

Because non normally negates the word or phrase that follows, and here it negates the verb:

• non mitteret = would not send

This is a very common and natural position in Latin.

Latin word order is much freer than English word order because the endings show the grammatical roles.

English depends heavily on position:

• the judge sends him

But Latin can move words around more easily because:

• iudex is nominative, so it is the subject
• eum is accusative, so it is the object
• carcerem is accusative after in, showing motion toward

This sentence is quite normal Latin:

• Si fur innocens esset = the if-clause first
• iudex eum in carcerem non mitteret = then the main clause

It is a present contrary-to-fact condition.

That means:

• the condition is imagined as untrue now
• the result is also imagined as untrue now

So the logic is:

• If the thief were innocent ...
• the judge would not send him to prison

But the form implies that in reality:

• the thief is not innocent
• the judge does send him to prison, or would do so

This is one of the most important Latin conditional patterns to learn.

In isolation, esset can often be translated in different ways depending on context, such as was or were. But in this sentence, because it is part of a contrary-to-fact conditional, were is the best English equivalent.

So here:

• esset = were
• not ordinary factual was

The grammar of the whole sentence determines the best translation.

They are:

• si = if
• fur, furis (m.) = thief
• innocens, innocentis = innocent
• sum, esse = to be
• iudex, iudicis (m./f.) = judge
• is, ea, id = he, she, it; that
• carcer, carceris (m.) = prison
• mitto, mittere, misi, missum = send

From those dictionary forms:

• esset comes from esse
• mitteret comes from mittere
• eum comes from is, ea, id
• carcerem comes from carcer

It would normally use the pluperfect subjunctive in both clauses.

For example:

• Si fur innocens fuisset, iudex eum in carcerem non misisset.

That means:

• If the thief had been innocent, the judge would not have sent him to prison.

So the contrast is:

• esset / mitteret = present contrary-to-fact
• fuisset / misisset = past contrary-to-fact