Learn Icelandic: Datt kötturinn niður í kjarrið? - Did the cat fall down into the brushwood?

The phrase niður í indicates movement towards a destination (down into). Because the cat is moving from one place to another (falling into the brushwood), the preposition í triggers the accusative case (kjarrið). If the cat were already resting down in the brushwood, you would use the dative case (kjarrinu).

What does adding the adverb niður before í do to the sentence?

It creates a compound directional phrase. The word niður emphasizes the downward trajectory of the fall. While í kjarrið alone simply means 'into the brushwood,' adding the directional adverb niður before the preposition is incredibly common in Icelandic to give a highly specific picture of the motion.

What gender is the noun kjarr, and why does it take the form kjarrið here?

The base word kjarr (brushwood or thicket) is a neuter noun. The -ið suffix is the attached definite article ('the') for the accusative case. Since neuter nouns look identical in the nominative and accusative cases, it remains kjarrið. If the preposition had forced the dative case, the word would be kjarrinu.

Do I always need to include niður when using the verb detta (to fall)?