Krukkan er opin, en lokið er enn á borðinu.

Because Icelandic usually puts the definite article, meaning the, onto the end of the noun instead of using a separate word.

• krukka = jar
• krukkan = the jar

Here -n is the definite ending. So Krukkan er opin means The jar is open.

Because the adjective has to agree with the noun in gender, number, and case.

Krukkan is:

• feminine
• singular
• nominative

So the adjective opinn changes to the feminine singular nominative form opin.

Compare:

• masculine: opinn
• feminine: opin
• neuter: opið

Even though opin comes after er, it still agrees with krukkan. This is very normal in Icelandic.

The basic noun is lok, meaning lid or cover.

Here it appears as lokið, which is the definite singular form:

• lok = lid
• lokið = the lid

This happens because lok is a neuter noun, and many neuter nouns form the definite nominative/accusative singular with -ið.

So lokið here means the lid.

They are different words:

• en = but
• enn = still, yet

So in this sentence:

• en connects the two clauses: but
• enn adds the idea of still: is still on the table

This is a very common pair for learners to confuse, because the spelling is so similar.

Because Icelandic uses different cases after á depending on whether you mean:

• location = where something is
• movement = where something is going

Here the lid is already resting on the table, so this is location, and á takes the dative.

• á borðinu = on the table
• á borðið = onto the table

So borðinu is the dative singular definite form of borð.

Because they do different jobs in the sentence.

In lokið er enn á borðinu:

• lokið is the subject of the clause, so it is in the nominative
• á borðinu is a prepositional phrase, and the preposition á requires the dative here because it expresses location

So only the noun inside the prepositional phrase changes case because of á. The subject does not.

In a neutral sentence like this, adverbs such as enn often come after the finite verb.

So:

• lokið er enn á borðinu

is a very natural word order.

The structure is roughly:

• lokið = subject
• er = verb
• enn = adverb
• á borðinu = prepositional phrase

Other word orders are possible in Icelandic for emphasis, but this is the most straightforward one.

Because Icelandic normally expresses the as a suffix attached to the noun.

So instead of having a separate word like English the, Icelandic often does this:

• krukka → krukkan
• lok → lokið
• borð → borðinu in the needed case

This is one of the big structural differences between English and Icelandic.

Here opin describes a state: the jar is open.

It does not specifically focus on the action of opening it. It just tells you the condition the jar is in right now.

That is very natural in Icelandic. In other words:

• Krukkan er opin = the jar is open

The sentence is about the situation now, not about who opened it or when.