Learn Hardcore Icelandic: Skápurinn í eldhúsinu er stór.

Icelandic doesn’t use a separate word for “the” as English does. Instead it attaches a definite-article suffix to the noun. Here skápur (cupboard) is a masculine noun, and adding -inn (definite, masculine, nominative, singular) gives skápurinn meaning “the cupboard.”

What case is Skápurinn in, and why?

Skápurinn is in the nominative case (singular). In Icelandic, the subject of a sentence always appears in the nominative. Since skápurinn is the subject (“the cupboard”), it takes the nominative ending.

Why is it í eldhúsinu and not just eldhús or eldhúsið?

í = “in,” a preposition that here shows location.
Static location with í requires the dative case.
eldhús (kitchen) is a neuter noun. Its definite nominative is eldhúsið but in the dative definite you get eldhús+inu = eldhúsinu.
So í eldhúsinu literally means “in the kitchen.”

What cases can the preposition í govern, and how do I know which one to use?

Dative (location): use when something is inside, at, on – e.g. í skólanum (“in the school”).
Accusative (movement): use when something is going into – e.g. ég fer í skólann (“I go into the school”).
Since our sentence says “the cupboard is in the kitchen,” we use dative: í eldhúsinu.

Why is the adjective stór not stórur or stórr?

Adjectives in Icelandic must agree in gender, number, and case with the noun they describe. Here stór is a predicative adjective (it follows er = “is”) and must be in masculine nominative singular to match skápurinn. For most adjectives the base (lexical) form is already the masculine nominative singular, so stór is correct.

What is the basic word order here? Why is er in third position?

Icelandic follows a V2 (verb-second) rule: the finite verb comes second in any main clause. In our sentence the first “constituent” is the entire noun phrase Skápurinn í eldhúsinu, so the verb er appears immediately after that, and then the predicate stór: Skápurinn í eldhúsinu | er | stór.
If you front the prepositional phrase instead, you still keep er second: Í eldhúsinu | er | skápurinn stór.

How would I say “A cupboard in a kitchen is big” (indefinite)?

Remove the definite endings: Skápur í eldhúsi er stór.
Here skápur is masculine nominative indefinite (“a cupboard”), eldhúsi is neuter dative indefinite (“in a kitchen”), and stór remains the predicate adjective in masculine nominative.

How do I pronounce á, í, and ú in this sentence?

á = roughly English “ow” as in “cow.”
í = long “ee” as in “see.”
ú = long “oo” as in “food.”
Approximate whole-sentence pronunciation:
SKOW-pur-rin ee-LTHOO-si-nu er STOHR.