Pöydällä on pari kirjaa.

Because this is an existential sentence in Finnish: it presents that something exists or is located somewhere.

A very common Finnish pattern is:

[place] + on + [thing]

So:

• Pöydällä on pari kirjaa.
• literally something like On the table is a couple of books
• natural English: There are a couple of books on the table.

If you start with kirjat, you get a more ordinary sentence about known books:

• Kirjat ovat pöydällä. = The books are on the table.

So the difference is not just word order. It also changes the focus:

• Pöydällä on pari kirjaa. = introducing or mentioning what is there
• Kirjat ovat pöydällä. = talking about specific books already known in the conversation

The ending -llä is the adessive case, and one of its common meanings is on or at.

So:

• pöytä = table
• pöydällä = on the table

The stem changes a little:

• pöytä → pöydä-
  • -llä

This is normal Finnish consonant gradation / stem change behavior.

The adessive often answers questions like:

• Where?
• On what?
• At what place?

More examples:

• tuolilla = on the chair
• kadulla = on the street
• asemalla = at the station

This is one of the most common things learners notice.

In this type of existential sentence, Finnish usually uses 3rd person singular:

• Pöydällä on kirja.
• Pöydällä on kirjat.
• Pöydällä on pari kirjaa.

So even if the thing mentioned is plural in meaning, the verb is often still on in this structure.

Compare:

• Pöydällä on pari kirjaa.
  = existential: There are a couple of books on the table.

• Ne kirjat ovat pöydällä.
  = ordinary sentence: Those books are on the table.

So:

• on is normal in this existential pattern
• ovat appears in a regular sentence where the subject is the main topic and agrees with the verb

Because pari is followed by the partitive singular.

So:

• kirja = book
• kirjaa = partitive singular of kirja

After pari, Finnish normally uses this pattern:

• pari kirjaa = a couple of books
• pari päivää = a couple of days
• pari euroa = a couple of euros

This is similar to how many quantity expressions work in Finnish.

So the structure is:

• pari
  • partitive singular noun

That is why kirjaa is correct here.

Because Finnish quantity expressions do not always work the way English does.

In English, you say:

• two books
• a couple of books

In Finnish, after numbers greater than one, and after words like pari, the noun is often in the partitive singular, not the plural.

For example:

• kaksi kirjaa = two books
• kolme autoa = three cars
• pari kirjaa = a couple of books

So even though the meaning is plural, the grammatical form of the noun is singular partitive.

This is very normal Finnish grammar.

It can mean either, depending on context.

Most often in everyday speech:

• pari means a couple of, often roughly two

Sometimes it can mean:

• exactly two

In many sentences, the difference is not very important. In Pöydällä on pari kirjaa, the natural idea is usually:

• there are a couple of books on the table
• not necessarily emphasizing the precise number in a mathematical way

If you want to be clearly exact, kaksi kirjaa is often more straightforward:

• Pöydällä on kaksi kirjaa. = There are two books on the table.

So:

• pari = often a little more casual
• kaksi = clearly the number two

Finnish has no articles like English a/an or the.

So Finnish does not have separate words corresponding directly to:

• a book
• the book

Instead, Finnish shows definiteness from:

• context
• word order
• sentence type
• sometimes pronouns like se, tämä, tuo

For example:

• kirja can mean a book or the book, depending on context
• pari kirjaa can mean a couple of books

In this sentence, the meaning is understood without articles.

The main reason here is pari.

The word pari requires the following noun to be in the partitive singular:

• pari kirjaa
• pari taloa
• pari minuuttia

There is also a broader Finnish pattern where the partitive is used with:

• quantities
• incomplete amounts
• unspecified portions
• ongoing or unbounded things in some contexts

But in this specific sentence, the easiest explanation is simply:

• pari takes partitive singular

So you do not need to look for a second hidden reason here.

Normally, no, because pöydässä means in the table, not on the table.

Compare:

• pöydällä = on the table
• pöydässä = in the table

So:

• Pöydällä on pari kirjaa. = There are a couple of books on the table.
• Pöydässä on reikä. = There is a hole in the table.

This is an important Finnish location contrast:

• -lla / -llä = on, at
• -ssa / -ssä = in

Yes, but the meaning or emphasis may change.

The most natural version here is:

• Pöydällä on pari kirjaa.

You might also hear:

• Pari kirjaa on pöydällä.

That version is possible, but it feels less like a neutral there are... sentence and more like you are talking specifically about a couple of books and saying where they are.

So, roughly:

• Pöydällä on pari kirjaa. = neutral existential statement
• Pari kirjaa on pöydällä. = focus more on the books

Finnish word order is flexible, but not random. Different orders often change the information structure.

A simple learner-friendly guide:

• Pöydällä ≈ PUY-dael-lae
• on ≈ on
• pari ≈ PA-ri
• kirjaa ≈ KEER-yaa

A few helpful points:

• ö is like a rounded vowel, somewhat like the vowel in French deux or German schön
• y is also rounded, somewhat like French u in tu
• ä is like the a in cat, but cleaner and shorter
• j in Finnish sounds like English y
  • so kirjaa sounds like kir-yaa, not kir-jaa with an English j
• double vowels are long:
  • aa is held longer than a

If you want a rough rhythm:

• PÖY-däl-lä on PA-ri KIR-jaa

In meaning, it is the thing being talked about as existing in that place, so yes, it functions like the subject of the existential sentence.

But Finnish existential sentences are a little special, so learners are often taught to think of the pattern as:

• place
  • on
    • what exists there

That is often easier than forcing it into the same mold as a basic English subject-verb sentence.

So in practical terms:

• Pöydällä = location
• on = is/are
• pari kirjaa = the thing that exists there

The most important thing for a learner is to recognize the pattern, even if the grammatical analysis feels slightly unusual compared with English.