Simul atque incendium ortum est, omnes ex aedificio fugerunt.

Here simul atque means as soon as.

A very literal sense of simul is at the same time, and atque means and or and then, but together Latin often uses simul atque idiomatically to introduce an event that happens immediately before another one:

• Simul atque incendium ortum est = As soon as the fire broke out

You may also see similar expressions such as simul ac.

The subject is incendium.

• incendium = fire
• It is in the nominative singular, so it is the thing that arose / broke out

So the clause works like this:

• incendium ortum est = the fire broke out

Because incendium is a neuter singular noun.

The verb here comes from orior, oriri, ortus sum, meaning to rise, to arise, or in this context to break out. In the perfect tense, this verb uses a participle plus sum:

• masculine singular: ortus est
• feminine singular: orta est
• neuter singular: ortum est

Since incendium is neuter, Latin uses the neuter form:

• incendium ortum est

So ortum agrees with incendium.

Because orior is a deponent verb.

Deponent verbs are verbs that have passive-looking forms but active meanings. So although ortum est looks like a passive perfect, it means:

• arose
• came into being
• broke out

not was arisen.

This is a very common thing in Latin, and orior is one of the standard examples.

Fugerunt is the 3rd person plural perfect active indicative of fugio, meaning to flee.

So:

• fugerunt = they fled or they have fled

In this sentence, English usually uses the simple past:

• everyone fled

The perfect tense is used because Latin is describing a completed event in the past.

Both clauses describe completed past actions:

• incendium ortum est = the fire broke out
• omnes ... fugerunt = everyone fled

Latin commonly uses the perfect in this kind of narrative sequence. The first event happens, and then the second follows immediately.

Because of simul atque, the first clause gives the trigger:

• As soon as the fire broke out, everyone fled

Aedificio is ablative singular.

The preposition ex means out of or from, and it takes the ablative case. So:

• ex aedificio = out of the building / from the building

The noun is:

• nominative: aedificium
• ablative: aedificio

That is why you see aedificio, not aedificium.

Omnes means all or everyone.

In this sentence it is being used substantively, meaning the noun is understood rather than stated:

• omnes = all the people / everyone

Grammatically, it is plural, so the verb is plural too:

• omnes ... fugerunt = everyone fled

English uses everyone with a singular form, but Latin often expresses this idea with a plural word meaning all people.

Because Latin does not have to match the English expression word for word.

English says:

• the fire broke out

But Latin uses the verb orior:

• incendium ortum est = literally something like the fire arose

In context, that is the normal Latin way to express a fire breaking out. So the idea of breaking out is built into the Latin verb phrase, not translated piece by piece.

The word order is fairly flexible, but the chosen order is natural and clear.

• Simul atque comes first to set up the time relationship immediately.
• incendium appears before ortum est, with the subject placed prominently.
• omnes comes early in the second clause to emphasize who reacted.
• ex aedificio is placed before fugerunt, which is a normal position for a prepositional phrase.

Latin relies more on endings than on word order to show grammatical relationships, so other orders would also be possible. But this order reads smoothly and highlights the sequence of events well.

Yes.

Ex can often be translated either way, depending on what sounds most natural in English:

• ex aedificio = from the building
• ex aedificio = out of the building

In this context, because people are fleeing during a fire, out of the building is often the most vivid translation. But grammatically, both are fine.