Mistökin kosta mikið.

The ending -in is the suffixed definite article, so Mistökin means the mistakes. Without the article, Mistök kosta mikið means Mistakes cost a lot in a general sense.

• Use the definite form when you mean specific, previously mentioned mistakes.
• For a general statement/gnomic truth, prefer the bare plural: Mistök kosta mikið or rephrase: Það er dýrt að gera mistök.

Mistök is a plural-only neuter noun (pluralia tantum). Icelandic typically says gera mistök even when English would say “make a mistake.” Examples:

• Ég gerði mistök = I made a mistake. If you need a singular-count noun, use villa (f.): Ég gerði villu = I made an error/a mistake. (This is less idiomatic than gera mistök, but it’s the singular word.)

Mistökin is nominative definite plural neuter of mistök. Quick overview:

• Nom/Acc: mistök (definite: mistökin)
• Dative: mistökum (definite: mistökunum)
• Genitive: mistaka (definite: mistakanna)

Agreement with a plural subject: Mistökin is plural, so the verb is kosta (3rd person plural present).

• 3sg: kostar (e.g., Bókin kostar…)
• 3pl: kosta (e.g., Mmistökin kosta…)

Here mikið is the neuter singular of the adjective mikill used adverbially, meaning “a lot” (a large amount). It’s not a separate object; it functions as a measure adverb with kosta.

• Use mikið with verbs to mean “a lot/much”: Þetta kostar mikið.
• Mjög means “very” and modifies adjectives/adverbs: Þetta er mjög dýrt. Saying kostar mjög is odd.

Yes:

• … kosta marga peninga (“… cost a lot of money,” with the count noun “money” in plural accusative).
• Or use a mass construction: … kosta mikið af peningum (“… cost a lot of money,” with af
  • dative: peningum). All of these are natural; kosta mikið alone already implies “a lot (of money).”

Invert verb and subject:

• Kosta mistökin mikið? You can also specify: Kosta þessi mistök mikið?

Yes:

• Mistök eru dýr. (“Mistakes are expensive.”) Note: dýr also means “animals,” but the meaning is clear in context.
• Very idiomatic: Það er dýrt að gera mistök. (“It’s expensive to make mistakes.”)

Approximate guide (stress on the first syllable of each word):

• Mistökin ≈ “MIS-tuh-kin” (ö is a rounded “uh,” like the vowel in “nurse” but rounded; final -in like “in”)
• kosta ≈ “KOS-ta”
• mikið ≈ “MEE-kith” (the k is fronted before i, and final ð is often like a soft voiceless th) A careful IPA approximation: [ˈmɪs.tœː.cɪn ˈkʰɔs.ta ˈmɪː.cɪθ].