Breakdown of Strákurinn bíður niðri í kjallaranum.
strákurinn
the boy
í
in / into / to (before the hour, in telling time) / per (in frequency expressions)
bíða
to wait
kjallarinn
the basement
Questions & Answers about Strákurinn bíður niðri í kjallaranum.
Why do we use niðri here instead of niður?
Niðri is used for position (being in a place), while niður is used for motion (moving down). Since the boy is just waiting and not actively moving downwards, we use the static position word niðri.
Does niðri literally mean "downstairs"?
Literally, it just means "down" (in a fixed position). However, when talking about a house or building, Icelanders use niðri to mean "downstairs" and uppi to mean "upstairs".
Why does kjallari (basement) change to kjallaranum here?
The preposition í (in) triggers the dative case when it describes a fixed location. Because the boy is stationary inside the basement, the word takes the dative definite ending -num. If he were moving into the basement, it would take the accusative case instead.
How do I know if bíður means "waits" or "is waiting"?
Icelandic doesn't have an exact match for the English "-ing" verb ending. The simple present tense bíður covers both "waits" and "is waiting". You just rely on the context of the sentence to know which one makes sense.