Leikarinn er mjög góður.

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

• leikari = actor
• leikarinn = the actor

The ending -inn here is the masculine singular definite article in the nominative form.

It actually is góður here because the adjective has to match the noun it describes.

Leikarinn is:

• masculine
• singular
• nominative

So the adjective also has to be:

• masculine
• singular
• nominative

That gives:

• masculine: góður
• feminine: góð
• neuter: gott

So Leikarinn er mjög góður uses góður because it agrees with leikarinn.

Er means is.

It is the present singular form of the verb vera, which means to be.

So:

• ég er = I am
• þú ert = you are
• hann/hún/það er = he/she/it is

In this sentence, leikarinn er means the actor is.

Mjög means very.

It modifies the adjective góður, so:

• góður = good
• mjög góður = very good

Its position is similar to English: it comes before the adjective it strengthens.

Because Icelandic often expresses the by attaching it to the noun as a suffix.

So instead of:

• the actor

Icelandic often says:

• leikarinn

This is one of the first big differences English speakers notice. Icelandic can use a separate determiner in some contexts, but the normal everyday definite form is usually attached to the noun.

Yes, in this sentence it is very similar to English:

• Leikarinn = subject
• er = verb
• mjög góður = complement

So the structure is basically:

The actor + is + very good

That said, Icelandic word order can be more flexible than English in other kinds of sentences.

A rough learner-friendly pronunciation is:

LAY-kar-inn er myoag GOH-thur

A few notes:

• ei in Leikarinn is pronounced somewhat like ay
• j in mjög sounds like English y
• ö does not exist in English; it is somewhat like the vowel in French peur or German schön
• ð in góður is like the th in this, not thin

Exact Icelandic pronunciation is a bit tricky, but this approximation can help at first.

Because here góður is not directly attached to the noun inside a noun phrase. It comes after the verb er and works as a predicate adjective.

Compare:

• góður leikari = a good actor
  Here the adjective comes before the noun.

• Leikarinn er góður = the actor is good
  Here the adjective comes after er.

This is very similar to English:

• a good actor
• the actor is good

It is in the nominative case, because it is the subject of the sentence.

The basic role breakdown is:

• subject = usually nominative
• leikarinn is the one being described
• therefore leikarinn is nominative

Because the noun is nominative masculine singular, the adjective góður also appears in the matching nominative masculine singular form.

Yes. Depending on context, góður can mean things like:

• good
• kind
• skilled
• nice
• worthwhile

In Leikarinn er mjög góður, the exact nuance depends on context. It could mean:

• the actor is morally good or kind
• the actor is a good actor / skilled performer

If the meaning has already been shown to the learner, the context probably makes clear which one is intended.