Spilastokkurinn er á borðinu.

Icelandic makes heavy use of compound nouns, much more than English does.

Spilastokkurinn is made up of:

• spila- = related to spil (cards, playing cards)
• stokkur = deck, stack
• -inn = the

So the whole word means the deck of cards.

This is very normal in Icelandic. Where English often uses several words, Icelandic often packs the idea into one compound noun.

In Icelandic, the is usually attached to the end of the noun instead of being a separate word.

So here:

• spilastokkurinn = the deck of cards
• borðinu = the table in the form required after á

That means both nouns in the sentence are definite.

Because the ending changes depending on:

• the noun’s gender
• its case
• its role in the sentence

Here:

• spilastokkurinn is masculine, singular, nominative, definite
• borðinu is neuter, singular, dative, definite

So the endings are different because the grammar is different.

A useful way to think about it is:

• -inn here marks the definite form of spilastokkur
• -inu is part of the definite dative singular form of borð

Because á can take different cases depending on meaning.

With location or position (on, at, staying somewhere), á usually takes the dative:

• á borðinu = on the table

With motion toward a surface (onto), á usually takes the accusative:

• á borðið = onto the table

So in your sentence, the deck is already resting there, not moving there, which is why Icelandic uses á borðinu.

It is dative singular definite.

Why? Because the preposition á takes the dative when it describes a fixed location.

So:

• borð = table
• borðinu = on the table / at the table, in the dative definite form

This is one of the most important preposition patterns to learn in Icelandic:

• dative for location
• accusative for movement toward something

Er means is.

It is the present singular form of the verb að vera = to be.

So:

• Spilastokkurinn er á borðinu.
• The deck of cards is on the table.

Unlike in some languages, you cannot leave out the verb here. Icelandic needs er just as English needs is.

Yes. The sentence you have is the most neutral order:

• Spilastokkurinn er á borðinu.

But Icelandic can also front the location for emphasis:

• Á borðinu er spilastokkurinn.

That still means the same thing, but it puts more focus on on the table.

One important rule: Icelandic is a verb-second language, so if another part comes first, the verb usually still stays in the second position.

So:

• correct: Á borðinu er spilastokkurinn
• not natural: Á borðinu spilastokkurinn er

If you want an indefinite version, you remove the definite endings:

• Spilastokkur er á borði. = A deck of cards is on a table.

You can also mix definite and indefinite nouns:

• Spilastokkur er á borðinu. = A deck of cards is on the table.
• Spilastokkurinn er á borði. = The deck of cards is on a table.

This is a good way to notice how much information Icelandic packs into noun endings.