الفتاة في المكتب.

A careful MSA reading is:

al-fatātu fī al-maktabi

A simple English-friendly approximation is:

al-fa-TAA-tu fee al-MAK-ta-bi

If you add the short vowels in writing, it would be:

الفتاةُ في المكتبِ

• الفتاةُ = al-fatātu
• في = fī
• المكتبِ = al-maktabi

Because Arabic usually does not use a present-tense verb meaning is/are/am in sentences like this.

So:

الفتاة في المكتب
literally looks like the girl in the office

but it means:

The girl is in the office

This is completely normal in Arabic. In the present tense, the linking verb is usually omitted.

ال is the Arabic definite article, equivalent to the.

So:

• الفتاة = the girl
• المكتب = the office

It appears on both words because both nouns are definite in this sentence.

A noun with ال does not take tanwīn, so you do not write something like الفتاةٌ or المكتبٌ.

في is a preposition meaning in.

So:

• في المكتب = in the office

Like English prepositions, it comes before the noun it governs. Unlike some very short Arabic particles such as بـ or لـ, في is written as a separate word.

The fully vocalized sentence is:

الفتاةُ في المكتبِ

Here is why:

• الفتاةُ is nominative because it is the subject of the sentence
• المكتبِ is genitive because it comes after the preposition في

So the grammar is:

• الفتاةُ — subject
• في المكتبِ — prepositional phrase functioning as the predicate

In everyday writing, these short vowels are usually omitted.

That letter is called tāʾ marbūṭa.

It often marks a feminine noun, and فتاة is indeed a feminine word meaning girl.

Important pronunciation note:

• In careful grammatical reading with endings, it is pronounced with a t sound before the case vowel: al-fatātu
• In pause, it is often heard more like -ah

So this ending is one common signal that a noun is feminine.

It is a full sentence in Arabic.

More specifically, it is a nominal sentence.

Its parts are:

• الفتاةُ — the subject, called mubtadaʾ
• في المكتبِ — the predicate, called khabar

Even though English needs is, Arabic does not here, so this is already a complete sentence.

Yes, in both words.

You say:

• al-fatātu
• al-maktabi

That is because ف and م are moon letters, so the l of ال is pronounced.

If the next letter were a sun letter, the l would assimilate instead. For example:

• الشمس is pronounced ash-shams, not al-shams

But in الفتاة and المكتب, the l stays audible.

Yes, but الفتاة في المكتب is the normal neutral order for this meaning.

You may also see the prepositional phrase first for emphasis:

في المكتب الفتاةُ

That is more marked, something like emphasizing in the office.

A very useful related pattern is:

في المكتب فتاةٌ

This means There is a girl in the office.

So changing the order can affect emphasis, and sometimes the meaning as well.