Málmurinn er þungur.

It is the definite article attached to the noun.

• málmur = metal
• málmurinn = the metal

In Icelandic, the is often not a separate word like in English. Instead, it is added to the end of the noun. Here, -inn is the definite ending for this noun in the masculine singular nominative form.

Málmur is the basic dictionary form of the noun, meaning metal. It is a masculine noun.

So:

• málmur = metal
• málmurinn = the metal

A learner often wants to know the gender because it affects adjective endings, article endings, and case forms.

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

Here, málmurinn is:

• masculine
• singular
• nominative

So the adjective also appears in the masculine singular nominative form:

• þungur = masculine singular nominative

If the noun were different, the adjective would change too:

• Bókin er þung. = The book is heavy.
  feminine singular
• Borðið er þungt. = The table is heavy.
  neuter singular
• Málmarnir eru þungir. = The metals are heavy.
  masculine plural

That is a very common question.

In this sentence, þungur is a predicate adjective: it comes after er and describes the subject.

After vera (to be), Icelandic normally uses the strong adjective form:

• Málmurinn er þungur. = The metal is heavy.

Even though málmurinn is definite, the adjective stays in the strong form here.

By contrast, if the adjective comes before the noun inside a noun phrase, you often get the weak form:

• þungi málmurinn = the heavy metal

So:

• Málmurinn er þungur. = full sentence, predicate adjective
• þungi málmurinn = noun phrase, adjective directly modifying the noun

Er is the 3rd person singular present tense of vera, the verb to be.

So it means is.

A few forms of vera are:

• ég er = I am
• þú ert = you are
• hann / hún / það er = he / she / it is
• við erum = we are
• þið eruð = you are
• þeir / þær / þau eru = they are

Here the subject is singular, so Icelandic uses er.

It is in the nominative case.

That is because it is the subject of the sentence:

• Málmurinn = the thing being talked about
• er þungur = is heavy

In simple statements like this, the subject is usually nominative. The adjective þungur also appears in the nominative because it agrees with the subject.

Yes. This is a normal Icelandic main-clause word order:

• Málmurinn = subject
• er = verb
• þungur = complement/adjective

So it works much like English:

• The metal is heavy.

Icelandic is often described as a verb-second language, which means the finite verb usually comes in the second position in main clauses. In this sentence, the subject comes first, so the verb naturally comes second.

A few useful pronunciation points:

• á sounds roughly like the vowel in now
• þ sounds like th in think
• stress is usually on the first syllable of the word

So very roughly:

• Málmurinn → stress on MÁL-
• þungur → starts with th as in think

A careful learner should especially notice:

• þ is never like the th in this; it is the voiceless sound, as in thin
• u in Icelandic is not exactly like English oo
• ng in þungur may sound like ng with a slight k quality after it

The exact sounds take practice, but those are the main things to watch.

Icelandic normally does not use a separate indefinite article like English a/an.

So málmur can mean:

• metal
• a metal

The exact meaning depends on context.

That means the contrast is usually:

• málmur = metal / a metal
• málmurinn = the metal

Not if you want to keep exactly the same sentence structure.

In Málmurinn er þungur, the adjective is part of the predicate: is heavy.

If you put the adjective before the noun, you are no longer saying The metal is heavy. You are making a noun phrase instead, like the heavy metal.

Compare:

• Málmurinn er þungur. = The metal is heavy.
• þungi málmurinn = the heavy metal

So yes, Icelandic can place the adjective before the noun, but that creates a different structure and often a different adjective form.

Everything that needs to agree would change:

• Málmarnir eru þungir. = The metals are heavy.

Changes:

• málmurinn → málmarnir
  singular definite → plural definite
• er → eru
  singular is → plural are
• þungur → þungir
  masculine singular adjective → masculine plural adjective

This is a good example of how Icelandic marks number on the noun, verb, and adjective.