Si custos portam non clausisset, fur ex urbe fugisset.

It is a past contrary-to-fact condition.

That means the sentence describes something that did not actually happen in the past.

So the Latin implies:

• the guard did shut the gate
• the thief did not escape from the city

This is the standard Latin pattern for if X had happened, Y would have happened.

Because Latin uses the pluperfect subjunctive in both parts of a past contrary-to-fact condition.

The pattern is:

• si
  • pluperfect subjunctive = the if-clause
• pluperfect subjunctive = the result clause

So here:

• Si custos portam non clausisset = if the guard had not shut the gate
• fur ex urbe fugisset = the thief would have fled from the city

The subjunctive here does not mean uncertainty in the ordinary sense; it marks the whole situation as unreal in the past.

No. Si by itself does not automatically require the subjunctive.

It can introduce different kinds of conditions:

• real/simple conditions often use the indicative
• future less vivid and contrary-to-fact conditions use the subjunctive

So the subjunctive here is not caused by si alone. It is caused by the fact that this is a contrary-to-fact past condition.

The two parts are:

• protasis = the if-clause
• apodosis = the main/result clause

In this sentence:

• Si custos portam non clausisset is the protasis
• fur ex urbe fugisset is the apodosis

These terms are useful because Latin grammar books often use them when explaining conditions.

Both are in the nominative singular, which is the normal case for the subject of a sentence.

• custos = the guard
• fur = the thief

Each one is the subject of its own clause:

• custos goes with clausisset
• fur goes with fugisset

Latin often leaves pronouns out, so nouns like these clearly identify who is doing the action.

Because portam is the direct object of clausisset.

The verb claudere means to shut/close, and it takes a direct object: you shut something.

So:

• porta = gate, in the nominative
• portam = gate, in the accusative

Here the guard is shutting the gate, so the accusative is required.

Because it follows the preposition ex, and ex takes the ablative.

• ex = out of, from
• urbs, urbis = city
• urbe = ablative singular

So ex urbe means out of the city or from the city.

This is a very common Latin pattern:

• ex
  • ablative
• ab
  • ablative
• cum
  • ablative

They are both third person singular active pluperfect subjunctive forms.

A useful way to think of the formation is:

• perfect stem + -isse-
  • personal ending

So:

• claudo, claudere, clausi, clausus → perfect stem claus- → clausisset
• fugio, fugere, fugi → perfect stem fug- → fugisset

These forms mean:

• he/she had shut in this conditional pattern
• he/she would have fled in this conditional pattern

In other words, the same Latin tense-form is used in both clauses, but English translates them differently depending on the clause.

Because English and Latin handle conditionals differently.

In Latin, a past contrary-to-fact condition usually has:

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

In English, we normally translate that as:

• if ... had ...
• ... would have ...

So:

• non clausisset → had not shut
• fugisset → would have fled

The difference is a matter of English idiom, not a difference in the Latin form.

It implies the opposite of both clauses.

Since it says:

• If the guard had not shut the gate, the thief would have fled from the city

the real situation must be:

• the guard did shut the gate
• the thief did not flee from the city

That is one of the most important things to notice in a contrary-to-fact condition.

No. Latin word order is much more flexible than English word order.

This sentence is perfectly normal, but Latin could rearrange it for emphasis. For example, the clauses could be reversed without changing the basic meaning.

What matters most is:

• the cases of the nouns
• the verb forms
• the relationship between the clauses

Also, non usually stands close to the word it negates, so non clausisset clearly means had not shut.

Not if the speaker wants this exact meaning.

If the speaker used the indicative, the sentence would no longer be a past contrary-to-fact condition. It would sound more like a simple or factual condition, depending on the forms used.

The subjunctive is essential here because it tells the reader:

• this is an unreal past situation
• the speaker is talking about what would have happened, not what actually happened

So the subjunctive is not optional in this construction.