في المبنى أمن.

In Arabic, a present-tense sentence often has no written verb meaning is. This is very common.

So في المبنى أمن literally looks like:

• in the building
• security / safety

But the natural meaning is something like There is security in the building or The building is safe/secure, depending on context.

Arabic does not need a separate word for is in this kind of present-tense statement.

في is the preposition in.

So:

• في المبنى = in the building

Like other Arabic prepositions, في makes the following noun genitive.

Because it has the definite article الـ, which means the.

So:

• مبنى = a building / building
• المبنى = the building

This sentence is talking about a specific building, or at least a building treated as known in the context.

The word المبنى is pronounced al-mabnā.

That final ى is alif maqṣūrah, and here it represents a long ā sound, not an ee sound.

So the end of the word sounds like:

• -nā

not

• -nī

This is a very common point of confusion for English speakers, because ى can look unusual at first.

Yes, أمن here is a noun. It usually means security, safety, or safeness, depending on context.

In this sentence, it is not an adjective like safe. It is more like the noun security/safety.

So the structure is roughly:

• In the building
  • there is security/safety

Because Arabic often uses an indefinite noun in this kind of existential sentence.

That is, the sentence is presenting the existence of something:

• في المبنى أمن = There is security in the building

If you made أمن definite, the meaning and feel would change. The indefinite form works naturally for introducing the idea that security/safety exists there.

In normal Arabic writing, short vowels and tanwīn are usually not written.

So the fully vocalized form would be:

• فِي الْمَبْنَى أَمْنٌ

That final ـٌ marks the noun as indefinite in the nominative, but in everyday writing it is usually omitted.

After the preposition في, المبنى is in the genitive case.

So grammatically it is majrūr.

However, this word is a special type of noun ending in ى, so the case ending is not obvious in normal spelling. Even when fully vocalized, nouns like this often do not show a visible change at the end the way many other nouns do.

So the important practical point is:

• في requires the genitive
• المبنى is therefore genitive here

A helpful way to understand it is:

• في المبنى = a prepositional phrase, in the building
• أمن = the thing being stated to exist, security/safety

So the sentence functions like an existential statement:

• There is security in the building

Traditional Arabic grammar may analyze this as a kind of nominal sentence with the prepositional phrase first and the noun after it. For a learner, the simplest idea is that Arabic can express there is/there are by using a phrase like in the building followed by an indefinite noun.

Yes, but the meaning or emphasis may change.

The version في المبنى أمن is very natural for emphasizing the location first:

• In the building, there is security

If you say أمن في المبنى, it is more like:

• Security is in the building

That is less natural in many contexts, unless you are contrasting locations or emphasizing security itself.

So the original order is a very normal way to express existence in a place.

It can mean either, depending on context.

أمن can refer to:

• security in the sense of protection, order, guardedness
• safety in the sense of absence of danger

So this sentence could suggest:

• the building is secure
• the building is safe
• there is security present in the building

The surrounding context would tell you which nuance is intended.