कुर्सियों के बीच एक छोटी मेज़ है।

Because कुर्सी changes form before a postposition.

The basic forms are:

• singular: कुर्सी
• plural: कुर्सियाँ
• plural oblique: कुर्सियों

After a postposition such as के बीच, Hindi uses the oblique form, so:

• कुर्सियों के बीच = between the chairs

So कुर्सियों is not a different word; it is the correct grammatical form of कुर्सी in this sentence.

के बीच is a very common Hindi expression meaning:

• between
• among
• in the middle of

In this sentence, कुर्सियों के बीच means between the chairs.

It is best learned as a chunk:

• X के बीच = between X / among X

Examples:

• दो घरों के बीच = between two houses
• लोगों के बीच = among people

Because the adjective छोटा must agree with मेज़ in gender and number.

मेज़ is usually treated as a feminine singular noun in Hindi, so:

• masculine singular: छोटा
• feminine singular: छोटी

That is why:

• छोटी मेज़ = small table

If the noun were masculine, you would use छोटा instead.

Yes, in standard Hindi मेज़ is usually feminine.

So you get:

• छोटी मेज़
• बड़ी मेज़
• नई मेज़

This is one of those nouns whose gender you mostly have to learn by usage. Even though मेज़ is a loanword, it still has grammatical gender in Hindi.

It can mean either, depending on context.

In many sentences, एक works like English a/an:

• एक छोटी मेज़ = a small table

But it can also mean the number one if the speaker wants to emphasize quantity:

• एक छोटी मेज़ = one small table

In this sentence, it most naturally feels like a small table, unless the context is emphasizing that there is exactly one.

Because Hindi usually puts the verb at the end.

A very literal word-for-word order here is something like:

• कुर्सियों के बीच = between the chairs
• एक छोटी मेज़ = a small table
• है = is

So the structure is:

• Between the chairs a small table is.

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

Hindi usually does not use a dummy subject like English there.

English says:

• There is a small table between the chairs.

Hindi simply says, more literally:

• Between the chairs a small table is.

So है by itself can express the idea of is / there is, depending on the sentence.

Because मेज़ is singular.

The verb agrees with the singular subject here:

• एक छोटी मेज़ है = there is one small table

If the noun were plural, you would use हैं:

• कुर्सियों के बीच छोटी मेज़ें हैं। = There are small tables between the chairs.

So:

• singular noun → है
• plural noun → हैं

Hindi has no articles like English a/an/the, so context decides.

That means कुर्सियों के बीच can be understood in English as:

• between chairs
• between the chairs

In natural translation, between the chairs is often the best choice, but Hindi itself does not mark the directly.

Yes. Hindi word order is flexible, especially for emphasis.

Both of these are natural:

• कुर्सियों के बीच एक छोटी मेज़ है।
• एक छोटी मेज़ कुर्सियों के बीच है।

The difference is mainly in focus:

• कुर्सियों के बीच... puts the location first
• एक छोटी मेज़... puts the table first

In either version, है normally stays near the end, and कुर्सियों के बीच stays together as a phrase.