फ्लैट में दस खिड़कियाँ हैं।

Hindi often puts the location first, then the thing that exists there, and then the verb.

So this sentence is literally close to:

In the flat ten windows are.

That is a very normal Hindi way to express there is/there are in a place.

Because में is a postposition, not a preposition.

In English, we say in the flat.
In Hindi, the equivalent word comes after the noun:

• फ्लैट में = in the flat
• कमरे में = in the room
• घर में = in the house

So Hindi uses noun + postposition, not preposition + noun.

Nouns before postpositions normally go into the oblique case in Hindi. But some nouns do not visibly change in the singular.

फ्लैट is a borrowed word ending in a consonant, so its singular direct and singular oblique forms look the same:

• फ्लैट
• फ्लैट में

Compare that with a noun that does change:

• कमरा = room
• कमरे में = in the room

So फ्लैट is in the right form; it just happens not to look different.

Because the sentence is talking about ten windows, so the noun must be plural.

The singular is:

• खिड़की = window

The direct plural is:

• खिड़कियाँ = windows

This is a common plural pattern for feminine nouns ending in -ी:

• लड़की → लड़कियाँ
• चाबी → चाबियाँ
• खिड़की → खिड़कियाँ

So दस खिड़कियाँ = ten windows.

The noun खिड़की ends in feminine -ी. In the direct plural, that ending changes to -ियाँ:

• खिड़की → खिड़कियाँ

A useful extra point:

• direct plural: खिड़कियाँ
• oblique plural: खिड़कियों

For example:

• दस खिड़कियाँ हैं = There are ten windows.
• खिड़कियों में = in the windows

So here खिड़कियाँ is used because the noun is plural and not directly followed by a postposition.

Because खिड़कियाँ is plural.

In the present tense:

• है is used with singular
• हैं is used with plural

Compare:

• फ्लैट में एक खिड़की है। = There is one window in the flat.
• फ्लैट में दस खिड़कियाँ हैं। = There are ten windows in the flat.

So the verb agrees with the plural noun खिड़कियाँ.

Because Hindi does not need a dummy word like English there in sentences like there is or there are.

English says:

• There are ten windows in the flat.

Hindi simply says:

• In the flat ten windows are.

So है/हैं can express existence without adding a separate word for there.

Hindi normally does not use articles like English a/an/the.

So फ्लैट can mean a flat or the flat, depending on context.

That means this sentence can be understood in English as something like:

• There are ten windows in the flat
• There are ten windows in a flat

Usually context tells you which one is meant.

No. दस stays the same.

Most basic Hindi numbers do not change form to match the noun’s gender. So you can say:

• दस लड़के = ten boys
• दस लड़कियाँ = ten girls
• दस खिड़कियाँ = ten windows

The noun changes as needed, but the number दस itself does not.

It is a very common loanword from English, and it is perfectly normal in modern Hindi, especially in urban contexts.

Depending on context, you might also hear:

• अपार्टमेंट
• घर
• मकान

But फ्लैट is very natural for an apartment/flat.

Yes, very often.

Hindi commonly expresses this idea with a location/existence structure:

• फ्लैट में दस खिड़कियाँ हैं।

That is the normal way to say that ten windows exist in the flat.

A learner might wonder about using के पास, but that is usually for possession in the sense of having something with someone, not for parts of a building:

• मेरे पास किताब है। = I have a book.

For a flat and its windows, में is the natural choice.

A helpful approximation is:

flat मेँ das khiṛkiyā̃ hain

More carefully:

• फ्लैट = flat
• में = mẽ (nasalized vowel)
• दस = das
• खिड़कियाँ = khiṛkiyā̃
• हैं = hain / hẽ

A few pronunciation notes:

• ड़ in खिड़की is a flap sound, often written ṛ
• ँ / nasalization appears in में, खिड़कियाँ, and हैं
• In natural speech, हैं often sounds closer to hẽ than a full English-style hain

So a practical learner-friendly pronunciation is:

flat mẽ das khiṛkiyā̃ hẽ