मेरे बैग में कलम है।

Because में is a postposition meaning in, and nouns before postpositions take the oblique case in Hindi.

So:

• direct form: मेरा बैग = my bag
• oblique form before में: मेरे बैग में = in my bag

This is very common in Hindi:

• मेरी किताब = my book
• मेरी किताब में = in my book
• मेरा घर = my house
• मेरे घर में = in my house

So मेरा changes to मेरे because of the postposition में.

में means in / inside.

Hindi uses postpositions, which come after the noun, unlike English prepositions, which usually come before.

So:

• बैग में = in the bag
• literally: bag in

This is normal Hindi structure.

Because Hindi uses postpositions rather than prepositions.

Compare:

• English: in the bag
• Hindi: बैग में = bag in

Other examples:

• घर में = in the house
• मेज़ पर = on the table
• स्कूल के पास = near the school

So the location word comes after the noun in Hindi.

Hindi usually follows Subject/Object + location + verb order, with the verb often coming at the end.

This sentence is structured like:

• मेरे बैग में = in my bag
• कलम = pen
• है = is

So the natural Hindi order is:

• In my bag pen is

That sounds odd in English, but it is normal in Hindi.

है is the present-tense form of the verb to be for singular things.

Here it tells you that the pen exists/is present in the bag.

So है can often mean:

• is
• exists
• there is, depending on the sentence

In this sentence, it works like there is.

Compare:

• कलम है। = There is a pen. / The pen is there.
• बैग में कलम है। = There is a pen in the bag.

Hindi usually does not use articles like a, an, or the the way English does.

So कलम can mean:

• a pen
• the pen

The exact meaning depends on context.

In this sentence, English normally translates it as a pen:

• There is a pen in my bag.

But Hindi itself does not need a separate article word here.

कलम is generally treated as feminine in standard Hindi.

That matters in some sentences, because adjectives and some verb forms can agree with feminine nouns.

However, in this sentence, the verb is है, which does not show masculine/feminine difference. So you do not see gender affecting the sentence much here.

You would notice it more in the plural:

• कलम है = there is a pen
• कलमें हैं = there are pens

Or with adjectives:

• अच्छी कलम = good pen (feminine form अच्छी)

Because मेरे belongs with बैग, not with कलम.

The phrase is:

• मेरे बैग में = in my bag

So मेरे is describing बैग, not कलम.

Also, because बैग is before a postposition (में), the possessive takes the oblique form मेरे.

So the structure is:

• मेरे बैग में = in my bag
• कलम है = there is a pen

Yes, in meaning, it is very close.

Hindi often expresses possession using a location-style structure:

• मेरे बैग में कलम है
• literally: In my bag, a pen is
• natural English: There is a pen in my bag
• close meaning: My bag has a pen in it

Hindi often prefers this kind of structure instead of directly saying that a thing has something.

Add नहीं before है:

• मेरे बैग में कलम नहीं है।
• There is no pen in my bag.

This is a very common pattern:

• घर में पानी है। = There is water in the house.
• घर में पानी नहीं है। = There is no water in the house.

You can make it a question in two common ways.

1. By intonation in speech:

   • मेरे बैग में कलम है?
   • Is there a pen in my bag?

2. By adding क्या at the beginning:

   • क्या मेरे बैग में कलम है?
   • Is there a pen in my bag?

Using क्या is clearer and very common in standard Hindi.

Yes. In everyday Hindi, many speakers use पेन very often.

So you may hear:

• मेरे बैग में पेन है।

That is completely natural in modern spoken Hindi.

The difference is mainly vocabulary choice:

• कलम = a more traditional Hindi/Urdu word
• पेन = the common borrowed English word

Both are understandable.

If there is more than one pen, you would usually say:

• मेरे बैग में कलमें हैं।
• There are pens in my bag.

Changes:

• कलम → कलमें for the plural
• है → हैं because the noun is plural

So:

• singular: मेरे बैग में कलम है।
• plural: मेरे बैग में कलमें हैं।

Yes, Hindi word order is somewhat flexible, but some orders are more natural than others.

Most natural:

• मेरे बैग में कलम है।

You might also hear:

• कलम मेरे बैग में है।

This puts more focus on the pen.

Both are grammatical, but the first one is the most neutral way to say There is a pen in my bag.

Primarily, it is a location/existence sentence:

• In my bag, there is a pen.

But Hindi often uses this kind of structure to express something very close to possession.

So it is both:

• literally: location/existence
• functionally: often similar to having

This is an important pattern in Hindi:

• मेरे पास पैसा है। = I have money.
  literally: Near me, money is.
• मेरे बैग में कलम है। = There is a pen in my bag. / My bag has a pen in it.

So the sentence is built as a location statement, even though its meaning overlaps with possession.