في المكتب ست كراسي.

A natural Egyptian pronunciation is fil-maktab sitt karāsi.

A quick breakdown:

• في المكتب → fil-maktab
• ست → sitt
• كراسي → karāsi

So the whole sentence sounds roughly like:

fil-MAK-tab sitt ka-RAA-si

In connected speech, في and المكتب flow together, so you usually hear fil-maktab, not two sharply separated words.

Because Arabic often leaves out the present-tense verb to be.

So where English says:

There are six chairs in the office

Arabic can simply say:

In the office, six chairs

That is normal grammar, not a shortened sentence.

In everyday Egyptian, people also often say في المكتب فيه ست كراسي, where فيه adds the idea of there is / there are more explicitly. But the version you have is still perfectly understandable.

Arabic often likes to start with the place, topic, or setting first.

So في المكتب sets the scene:

In the office ...

Then the new information comes:

... six chairs

This is very natural in Arabic. It is a bit like saying in English:

In the office, there are six chairs.

If you switch the order to ست كراسي في المكتب, that can also work, but the focus feels a little different.

Here it means in.

That is because it is followed directly by a place noun:

• في المكتب = in the office

In Egyptian Arabic, في can also be used in an existential sense, like there is / there are, but that is a different use. Context tells you which one is meant.

In this sentence, في المكتب is clearly a prepositional phrase meaning in the office.

ال is the Arabic definite article, meaning the.

So:

• مكتب = office or desk
• المكتب = the office or the desk

After في, the words are connected in speech:

• في المكتب
• pronounced smoothly as fil-maktab

This happens because the l of ال joins naturally to the end of في.

Also, م is a moon letter, so the l sound stays. That is why you hear l in fil-maktab.

In Egyptian Arabic, numbers often appear in a shorter colloquial form before nouns.

So ست كراسي is a very normal Egyptian way to say:

six chairs

If you know some Standard Arabic, you may expect ستة كراسي. That is the more formal type of form you often see in MSA. In everyday Egyptian speech, the shorter ست is very common.

So for a learner, the main thing to notice is:

• ست here is a normal Egyptian counting form
• it means six

Because the sentence is introducing some chairs, not referring to a specific already-known set of chairs.

So:

• ست كراسي = six chairs
• الست كراسي = the six chairs

In existential or descriptive sentences like this one, Arabic usually uses an indefinite noun phrase, just like English says six chairs, not the six chairs.

The singular is كرسي = chair.

The plural is كراسي = chairs.

This is a broken plural, which means Arabic changes the internal pattern of the word instead of just adding a simple ending.

So:

• كرسي → chair
• كراسي → chairs

This is very common in Arabic, and it is something learners need to get used to.

Yes, both are possible.

1. في المكتب فيه ست كراسي

This is very natural in everyday Egyptian and explicitly includes فيه = there is / there are.

It feels like:

In the office, there are six chairs.

2. ست كراسي في المكتب

This is also possible, but the focus is a little more on six chairs.

3. في المكتب ست كراسي

This is the version you were given. It is concise, natural, and very common in simple description.

So the sentence can be phrased in more than one way, with small differences in emphasis.