Aðalsetningin er stutt, en aukasetningin er lengri.

Both are compound nouns:

• aðalsetningin = aðal
  • setning
    • -in
• aukasetningin = auka
  • setning
    • -in

Here:

• setning means sentence or clause, depending on context
• aðal- means main / principal
• auka- means additional / subordinate in this grammar term
• -in is the attached definite article, meaning the

So the structure is very literally:

• aðalsetningin = the main clause
• aukasetningin = the subordinate clause

They are mainly grammar terms.

A native speaker would use them when talking about sentence structure, in school, grammar books, or language discussions. They are not unusual words, but they are more technical than everyday conversational vocabulary.

Because Icelandic usually adds the definite article to the end of the noun instead of putting a separate word in front, like English the.

So:

• aðalsetning = main clause
• aðalsetningin = the main clause

and

• aukasetning = subordinate clause
• aukasetningin = the subordinate clause

This is very common in Icelandic. The article is often called a suffixed article.

Both are feminine.

The key is the last part of the compound: setning is a feminine noun, and compound nouns normally take their gender from the final element.

So:

• aðalsetning is feminine
• aukasetning is feminine

That matters because adjectives describing them must agree with them.

Good question. In this sentence, stutt is the correct form with aðalsetningin.

The adjective comes from stuttur = short. Its forms include:

• masculine: stuttur
• feminine: stutt
• neuter: stutt

So stutt is not only neuter here; it is also the correct feminine singular form.

Also, after er (is), Icelandic adjectives still agree with the subject:

• Aðalsetningin er stutt. = The main clause is short.

Because lengri is the comparative form of langur (long), so it means longer.

The sentence is contrasting the two clauses:

• the main clause is short
• the subordinate clause is longer

So lengri shows comparison.

A useful pattern is:

• langur = long
• lengri = longer
• lengstur = longest

Here, en means but.

It joins two full clauses:

• Aðalsetningin er stutt
• aukasetningin er lengri

So the sentence means:

• The main clause is short, but the subordinate clause is longer.

One thing to watch out for: en can also mean than after a comparative. So Icelandic learners often need to decide from context which meaning it has.

Because Icelandic, like English, does not always need to state the second half of the comparison explicitly.

Here, the comparison is understood from the contrast:

• one clause is short
• the other is longer

It is clear that longer means longer than the main clause.

If you wanted to say it more explicitly, you could say:

• Aukasetningin er lengri en aðalsetningin.
• The subordinate clause is longer than the main clause.

In that sentence, en means than, not but.

Because en is joining two coordinated clauses, and Icelandic commonly uses a comma before en in this kind of sentence.

So the structure is:

• clause 1: Aðalsetningin er stutt
• comma
• en
• clause 2: aukasetningin er lengri

This punctuation is normal and standard.

Both nouns are in the nominative case because they are the subjects of their clauses:

• Aðalsetningin = subject of er stutt
• aukasetningin = subject of er lengri

The predicate adjectives also match the subject in gender and number, and in this kind of sentence they appear in the appropriate agreeing form.

No, this is a very normal, neutral word order.

Each clause is basically:

• subject + er + adjective

So:

• Aðalsetningin er stutt
• aukasetningin er lengri

That is the most straightforward way to say it.

You could change the order for emphasis in some contexts, but this version is the standard one a learner should be comfortable with first.

Because Icelandic does not capitalize all nouns the way German does.

In Icelandic, you normally capitalize:

• the first word of the sentence
• proper names

So in this sentence only Aðalsetningin is capitalized because it comes first, not because it is a noun. If aukasetningin came at the start of a sentence, it would be capitalized there too.