Glæpurinn var alvarlegur.

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

• glæpur = crime
• glæpurinn = the crime

So -inn here is the masculine singular nominative definite article.

Glæpur is a masculine noun. That matters because adjectives in Icelandic must agree with the noun they describe in:

• gender
• number
• case

Since glæpurinn is masculine, singular, and nominative here, the adjective also has the matching form: alvarlegur.

The -ur ending shows that the adjective is in the masculine singular nominative form.

In this sentence:

• glæpurinn = masculine singular nominative
• alvarlegur = masculine singular nominative

This is standard Icelandic agreement. English adjectives do not change form this way, but Icelandic adjectives do.

Var is the past tense of vera (to be).

• er = is
• var = was

So:

• Glæpurinn er alvarlegur. = The crime is serious.
• Glæpurinn var alvarlegur. = The crime was serious.

Both are in the nominative here.

• glæpurinn is the subject, so nominative is expected.
• After the verb vera (to be), the predicate adjective normally also appears in the nominative and agrees with the subject.

So Icelandic treats this as:

• subject in nominative
• predicate adjective also in nominative

No. Icelandic often expresses the by attaching it to the noun.

So English:

• the crime

becomes Icelandic:

• glæpurinn

There is no separate standalone the here.

A rough English-friendly guide would be:

• Glæpurinn ≈ GLYE-pu-rin
• var ≈ var
• alvarlegur ≈ AL-var-ley-gur

A few helpful notes:

• æ is roughly like eye
• rolled or tapped r is common in Icelandic
• the stress is usually on the first syllable: GLÆpurinn, ALvarlegur

This is only an approximation, but it helps get started.

This is the normal straightforward pattern:

• subject
  • verb
    • complement

So:

• Glæpurinn = subject
• var = verb
• alvarlegur = predicate adjective/complement

It matches the basic English order The crime was serious quite closely.

Not in this sentence as written, because alvarlegur is not directly attached to the noun here. It is part of the predicate after var.

Compare:

• alvarlegur glæpur = a serious crime
  Here the adjective comes directly before the noun.

But in your sentence:

• Glæpurinn var alvarlegur = The crime was serious
  Here the adjective comes after was, just like in English.

You would say:

Glæpurinn var ekki alvarlegur.

• ekki = not

So the structure is:

• Glæpurinn = the crime
• var = was
• ekki = not
• alvarlegur = serious

Yes. Icelandic adjectives change form to match the noun.

For example, in the singular nominative:

• masculine: alvarlegur
• feminine: alvarleg
• neuter: alvarlegt

So the form alvarlegur is specifically the masculine one, which matches glæpurinn.

The indefinite version is:

Glæpur var alvarlegur.

That means A crime was serious or Crime was serious, depending on context, though in natural English you would usually need an article or a more specific context.

The main difference is:

• glæpur = crime
• glæpurinn = the crime

It helps to recognize both the whole form and its parts.

You should understand it as:

• glæpur = noun stem/dictionary form
• -inn = definite article ending

So yes, learn glæpurinn when you meet it, but also notice the pattern. That will help you recognize many other Icelandic definite nouns too.