Skattframtalið fyrir tekjuárið er opið.

Because Icelandic usually puts the definite article on the end of the noun instead of using a separate word like English the.

• skattframtal = tax return

• skattframtalið = the tax return

• tekjuár = income year / tax year
• tekjuárið = the income year / the tax year

So the ending -ið here is the neuter singular definite article.

They are in different cases:

• Skattframtalið is the subject of the sentence, so it is in the nominative.
• tekjuárið comes after the preposition fyrir, which here takes the accusative.

A useful thing to know is that for many neuter nouns, the nominative and accusative singular look the same. So even though tekjuárið is accusative here, it has the same form as nominative tekjuárið.

Because the adjective has to agree with the noun it describes.

Skattframtal is a neuter singular noun, so the adjective is also neuter singular:

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

So:

• skattframtalið er opið = the tax return is open

Also, after vera (to be), Icelandic normally uses the strong adjective form in this kind of predicate sentence, even if the noun is definite.

Compare:

• Húsið er stórt = the house is big
• Skattframtalið er opið = the tax return is open

Yes. It is a compound:

• skatt- = tax
• framtal = return, declaration, statement

So skattframtal literally means something like tax declaration or tax return.

A very important rule in Icelandic compounds is that the last part usually determines the grammatical gender of the whole word. Since framtal is neuter, skattframtal is neuter too.

That is why you get:

• skattframtalið
• opið

both in neuter singular forms.

This is another compound word:

• tekja = income, revenue
• ár = year

In compounds, Icelandic often uses a linking form, very often based on a genitive form. Here the first part appears as tekju-, giving:

• tekjuár = income year / tax year

Then the definite article is added:

• tekjuárið = the income year

So the structure is:

tekju- + ár + -ið

This kind of formation is extremely common in Icelandic compounds.

It is the normal word for open, but here it is being used in a broader, administrative sense.

So er opið does not mean that a physical object is standing open. It means something more like:

• available
• active
• open for submission
• open for filing

This is very natural Icelandic for online systems, application periods, forms, registrations, and similar official processes.

Because Icelandic main clauses normally follow the verb-second pattern.

In this sentence, the first slot is occupied by the subject phrase:

• Skattframtalið fyrir tekjuárið

Then the finite verb comes second:

• er

Then the rest follows:

• opið

So the structure is basically:

[subject phrase] + [verb] + [predicate]

If you move something else to the front, the verb still tends to stay in second position. For example:

• Núna er skattframtalið fyrir tekjuárið opið.
  = Now the tax return for the income year is open.

That verb-second pattern is a key feature of Icelandic sentence structure.

Grammatically you can make indefinite versions of these nouns, but in this sentence the definite forms are much more natural.

Why? Because the sentence is talking about a specific, known thing:

• the tax return for a particular year
• the filing period for that specific return

So skattframtalið fyrir tekjuárið sounds like the tax return for the income year, which fits the situation.

If you remove the definite article, the sentence becomes more general and less natural in this kind of official announcement. It would sound more like you are talking about tax returns in general, not the specific one people are expected to file now.