Á borðinu eru tvö avókadó og ein sítróna.

It is built into borðinu.

Icelandic usually puts the definite article onto the end of the noun instead of using a separate word:

• borð = table
• borðið = the table
• borðinu = the table in the dative case

So Á borðinu means on the table.

Because á can take different cases depending on meaning.

Here it means a fixed location, so it takes the dative:

• á borðinu = on the table

If there were movement onto the table, Icelandic would usually use the accusative:

• á borðið = onto the table

So the contrast is roughly:

• Það er á borðinu = it is on the table
• Ég set það á borðið = I put it onto the table

Because Icelandic often puts the location first when that is the setting or topic.

So Á borðinu eru... is a very natural way to say that something is on the table. It is similar to English patterns like:

• On the table, there are...

It helps present the scene first, and then the things being talked about.

This is because Icelandic main clauses usually follow a verb-second pattern.

That means the finite verb normally comes in the second position. Here the first element is Á borðinu, so the verb comes next:

• Á borðinu
  • eru
    • tvö avókadó og ein sítróna

If the subject came first, you could also say:

• Tvö avókadó og ein sítróna eru á borðinu

That is also correct, but the focus is a bit different.

Because the subject is plural overall.

Even though one item is ein sítróna and the other is tvö avókadó, together they make more than one thing, so Icelandic uses the plural of vera:

• er = is
• eru = are

So:

• Sítrónan er á borðinu = The lemon is on the table
• Tvö avókadó og ein sítróna eru á borðinu = The two avocados and one lemon are on the table

Because avókadó is a neuter noun, and the numeral two agrees with the gender of the noun.

In the nominative:

• tveir = two, masculine
• tvær = two, feminine
• tvö = two, neuter

So:

• tveir bílar = two cars
• tvær bækur = two books
• tvö avókadó = two avocados

Because sítróna is a feminine noun.

The word for one changes by gender in Icelandic:

• einn = one, masculine
• ein = one, feminine
• eitt = one, neuter

Since sítróna is feminine, you use ein:

• ein sítróna = one lemon

Because ein sítróna is part of the subject, so it stays in the nominative case.

The word order may make it look unusual to an English speaker, because the subject comes after the verb here, but it is still the subject:

• Á borðinu eru tvö avókadó og ein sítróna

If sítróna were an object, then you might see another case form, such as sítrónu in the accusative:

• Ég sé eina sítrónu = I see one lemon

So the form depends on grammatical role, not just on where the word appears in the sentence.

Because some Icelandic nouns, especially newer loanwords, can have the same form in singular and plural in some cases.

So:

• avókadó can mean an avocado
• avókadó can also be the plural form here after tvö

The numeral tells you the number clearly, so there is no confusion:

• eitt avókadó = one avocado
• tvö avókadó = two avocados

No. Icelandic does not have an indefinite article like English a/an.

A bare noun can often mean a noun:

• sítróna = lemon / a lemon

Here ein sítróna specifically emphasizes the number one. That makes sense because the sentence is counting items:

• tvö avókadó og ein sítróna

So ein is not just an article here; it is a real numeral.

Yes. A few natural options are possible, depending on what you want to emphasize.

For example:

• Tvö avókadó og ein sítróna eru á borðinu.
• Á borðinu eru tvö avókadó og ein sítróna.

Both are correct.

Very roughly:

• Á borðinu eru... puts the location first
• Tvö avókadó og ein sítróna eru... puts the things first

You may also see existential-style sentences such as:

• Það eru tvö avókadó og ein sítróna á borðinu.

That is also possible, though the original sentence is very natural and common.