أنا داخل المبنى.

A careful MSA pronunciation is:

ʾanā dākhil al-mabnā

A rough breakdown:

• أنا = ʾanā
• داخل = dākhil
• المبنى = al-mabnā

A few pronunciation notes:

• خ in داخل is the throaty sound heard in names like Khalid.
• ال in المبنى is pronounced al- because م is a moon letter, so the l does not disappear.
• The last sound in المبنى is ā, not y.

Because in Arabic, the present-tense verb to be is usually omitted in simple statements.

So Arabic says:

• أنا داخل المبنى

where English says:

• I am inside the building

This is completely normal in MSA.
If you wanted a past or future meaning, then Arabic would use a verb, for example:

• كنتُ داخلَ المبنى = I was inside the building
• سأكون داخلَ المبنى = I will be inside the building

Here is the basic breakdown:

• أنا = the subject, I
• داخل المبنى = the predicate, telling where the speaker is

More specifically:

• داخل = inside / the interior of
• المبنى = the building

So the structure is essentially:

• I + inside the building

In this sentence, داخل is best understood as a location word meaning inside or the interior of.

It is not a verb here.
It is part of the expression داخل + noun, such as:

• داخل البيت = inside the house
• داخل الغرفة = inside the room
• داخل المبنى = inside the building

So in أنا داخل المبنى, it is functioning as the location part of the sentence.

Because داخل already carries the meaning inside.

So:

• أنا داخل المبنى

already means I am inside the building.

You do not need في here.

You may sometimes see a fuller expression like:

• أنا في داخل المبنى

but that is more expanded and less basic. For a learner, أنا داخل المبنى is perfectly normal.

Yes, أنا في المبنى is also correct.

The difference is mainly one of emphasis:

• أنا في المبنى = I am in the building
• أنا داخل المبنى = I am inside the building

In many situations they are very close in meaning, but داخل makes the idea of being inside more explicit.

Because the sentence is talking about the building, not just a building.

So:

• المبنى = the building

This matches the English meaning with a specific building already understood from context.

Also, after داخل, the noun normally comes directly after it:

• داخل المبنى
• داخل البيت
• داخل المدرسة

Because داخل commonly links directly to the following noun.

This is a very common Arabic pattern:

• داخل البيت
• داخل الغرفة
• داخل المبنى

So Arabic does not need an extra linking word between داخل and المبنى.
Together they form one location phrase: inside the building.

That letter is called alif maqṣūra.

It looks like a dotless ي, but it is pronounced as a long ā sound.

So:

• المبنى is pronounced al-mabnā

not al-mabniy and not al-mabnī.

This is a very common spelling pattern in Arabic.

Usually, in this kind of sentence, learners can treat داخل as a fixed location word meaning inside, so it does not have to change.

That means the pattern داخل + noun is the important thing:

• أنا داخل المبنى
• هي داخل المبنى

In other contexts, داخل can behave differently, and in non-MSA usage you may hear agreement more often. But for standard beginner-level MSA, the location expression داخل + noun is the key idea.

As a full neutral sentence, أنا is the normal form because it clearly gives the subject: I.

However, داخل المبنى can work by itself as a short answer in context.

For example, if someone asks:

• أين أنت؟ = Where are you?

you could answer briefly:

• داخل المبنى = Inside the building

So yes, it can be omitted in context, but not usually in a full stand-alone sentence for beginners.

In normal everyday Arabic writing, case endings are usually not written, so you normally just see:

• أنا داخل المبنى

In fully vocalized grammatical MSA, داخل is often analyzed here as an adverb of place, so many grammarians would read it as:

• أَنا داخلَ المبنى

The final noun is then understood in the genitive after داخل.

For most learners, the main thing to remember is this:

• in ordinary text, you usually will not see those endings written
• the practical pattern to learn is أنا داخل المبنى