Ruina et incendium pericula sunt.

Because the sentence has a compound subject: ruina et incendium = collapse and fire.

In Latin, when two singular nouns are joined by et (and), they usually take a plural verb. So:

• ruina = collapse
• incendium = fire
• ruina et incendium = collapse and fire
• sunt = are

So sunt is plural because two things are being talked about together.

Pericula is plural because it matches the idea of the compound subject.

The sentence is saying that collapse and fire are dangers, not is a danger. Since there are two subject items joined by and, Latin uses the plural predicate noun:

• singular: periculum = danger
• plural: pericula = dangers

So pericula sunt means are dangers.

Because periculum is a neuter noun.

In Latin, many second-declension neuter nouns have:

• singular nominative: -um
• plural nominative: -a

So:

• periculum = danger
• pericula = dangers

This is very common for neuter nouns:

• bellum = war → bella = wars
• donum = gift → dona = gifts
• incendium = fire → incendia = fires

So even though -a may look singular to an English speaker, here it is a neuter plural ending.

All three main nouns are in the nominative:

• ruina — nominative singular
• incendium — nominative singular
• pericula — nominative plural

Why?

• ruina and incendium are the subject
• pericula is a predicate nominative after sunt

With forms of to be such as est and sunt, Latin uses the nominative for both the subject and the noun that identifies or describes the subject.

So this is literally structured as:

collapse and fire dangers are

Yes. Together they form a compound subject.

Latin often works like English here:

• Marcus et Julia veniunt = Marcus and Julia come
• Ruina et incendium pericula sunt = Collapse and fire are dangers

Each noun is singular by itself, but together they make a plural idea, which is why the verb is plural: sunt.

Because Latin has no articles.

English distinguishes between:

• a collapse
• the collapse
• collapse

Latin does not have separate words for a/an/the. The exact sense depends on context.

So ruina et incendium pericula sunt could be understood in English in slightly different ways depending on context, such as:

• Collapse and fire are dangers
• A collapse and a fire are dangers
• The collapse and the fire are dangers

In a basic grammar example, Collapse and fire are dangers is the most natural translation.

Because Latin word order is more flexible than English word order.

English usually requires something like:

Collapse and fire are dangers.

Latin often likes to place the verb later, especially at the end:

Ruina et incendium pericula sunt.

That does not change the meaning. Latin endings do much of the grammatical work, so word order is often used for style, emphasis, or rhythm rather than basic grammar.

Yes, often they could.

For example, these would still basically mean the same thing:

• Pericula sunt ruina et incendium.
• Incendium et ruina pericula sunt.
• Ruina et incendium sunt pericula.

However, different word orders may sound more or less natural depending on context, and they can shift emphasis slightly.

The original order is a very normal Latin way to state the idea.

They belong to two different declensions:

• ruina — first declension
• incendium — second declension neuter
• periculum — second declension neuter (here seen in the plural pericula)

So you can parse them like this:

• ruina = nominative singular, first declension
• incendium = nominative singular, second declension neuter
• pericula = nominative plural, second declension neuter

This sentence is useful because it shows you nouns of different declensions working together in one simple statement.

Et means and.

It simply joins ruina and incendium:

• ruina et incendium = collapse and fire

This creates the compound subject of the sentence.

You usually understand it from the meaning and the structure of the sentence.

Grammatically, Latin sentences with to be can sometimes be read in either direction because both nouns are in the nominative. But here the most natural reading is:

• ruina et incendium = the things being talked about
• pericula = what they are called

So the sentence means:

Collapse and fire are dangers

rather than the less natural:

Dangers are collapse and fire

That said, because Latin is flexible, context can sometimes affect which side is felt more strongly as the subject.

It is singular here.

Even though English ruins is often plural, Latin ruina in this sentence is a singular noun meaning something like collapse, fall, or ruin in the sense of a destructive downfall.

So:

• ruina = a collapse / ruin
• not plural ruinae

Because Latin is listing two singular things joined by and, not making each noun plural.

So the sentence is:

• ruina = collapse
• incendium = fire

Together: collapse and fire

That compound idea is plural, so the verb is plural (sunt) and the predicate noun is plural (pericula), but the two individual items being listed remain singular.