मेरा बैग कुर्सी के नीचे है।

Because Hindi usually puts the main verb at the end. So the structure is more like:

मेरा बैग = my bag
कुर्सी के नीचे = under the chair
है = is

A very literal English-style rearrangement would be My bag under the chair is, but normal English changes the order to My bag is under the chair.

मेरा agrees with बैग, not with the speaker.

In Hindi, words like मेरा / मेरी / मेरे change to match the thing being possessed:

• मेरा = masculine singular
• मेरी = feminine singular
• मेरे = plural or masculine oblique in some contexts

Since बैग is usually treated as a masculine singular noun, you say मेरा बैग.

Compare:

• मेरा बैग = my bag
• मेरी किताब = my book
• मेरे जूते = my shoes

It means under the chair.

Hindi uses postpositions rather than prepositions. That means the location word comes after the noun, not before it.

So instead of English under the chair, Hindi says:

• कुर्सी = chair
• के नीचे = under / below

Together: कुर्सी के नीचे = under the chair

This is a very common question. In expressions like के नीचे, के पास, के बाद, के सामने, the के is part of a fixed compound postposition.

So for a beginner, the safest way is to learn:

• X के नीचे = under X
• X के पास = near X
• X के बाद = after X

You do not normally switch this to की नीचे just because the noun is feminine. So:

• कुर्सी के नीचे = under the chair
• मेज़ के नीचे = under the table
• घर के नीचे = under the house/building

The noun before a postposition usually goes into the oblique case. But with some nouns, the oblique form looks the same as the basic form.

That is what happens here:

• direct form: कुर्सी
• oblique singular: कुर्सी

So even though it is grammatically in the oblique, it still looks the same.

You can see a clearer change in the plural:

• कुर्सियाँ = chairs
• कुर्सियों के नीचे = under the chairs

है means is.

So the sentence is a standard present-tense statement of location:

• मेरा बैग कुर्सी के नीचे है। = My bag is under the chair.

In standard Hindi, है should be included here. In very casual speech, people sometimes drop forms of to be, but learners should normally keep it.

If the subject were plural, you would use हैं instead:

• मेरे बैग कुर्सी के नीचे हैं। = My bags are under the chair.

Hindi does not have articles like a, an, and the.

So Hindi often leaves that idea to context. Depending on the situation, कुर्सी could be understood as:

• a chair
• the chair

In natural English, this sentence is usually translated as My bag is under the chair, but Hindi itself does not mark the explicitly.

It is a very common English loanword used in Hindi. Everyday Hindi uses many such borrowed words.

So बैग sounds completely normal in conversation.

You could also use more traditional Hindi words in some contexts, such as:

• थैला
• झोला (depending on the kind of bag)

But मेरा बैग कुर्सी के नीचे है is perfectly natural modern Hindi.

The main things that may change are:

1. the possessive word
2. the verb
3. sometimes the noun before the postposition

Examples:

• मेरी किताब कुर्सी के नीचे है। = My book is under the chair.
  (मेरी because किताब is feminine singular)

• मेरे बैग कुर्सी के नीचे हैं। = My bags are under the chair.
  (मेरे and हैं because बैग is plural)

• मेरा बैग कुर्सियों के नीचे है। = My bag is under the chairs.
  (कुर्सियों is the oblique plural form of कुर्सियाँ)

So this one sentence is a good example of several Hindi patterns working together.