Time, Space, and Numbers in Icelandic

In Icelandic, cases (accusative, dative, genitive) aren't just for the people doing actions. We also use them to build "adverbial phrases" (aukafallsliðir). These phrases tell us when, where, or how much something is happened.

Here is how to choose the right case for these descriptions.

Aukafallsliðir: How to use Cases for Details

Tími (Time)

When talking about time, the case changes depending on whether you are stating a date or comparing times.

The Specific Date (Þolfall / Accusative): When stating a specific date or day, use the Accusative.

Jón kom heim fyrsta júní. (Jón came home on June 1st.)
Skólinn byrjar tuttugasta ágúst. (School starts on August 20th.)

The Comparison (Þágufall / Dative): When strictly comparing one time to another (saying "X days later" or "X years ago"), use the Dative.

Hann fór aftur þremur vikum seinna. (He went back three weeks later.)
Kennararnir mæta tveimur dögum fyrr. (The teachers arrive two days earlier.)

Staður (Place / Addresses)

Usually, place names follow prepositions (í, á, til). However, when giving a specific street address in a sentence, it often stands alone in the Dative case.

The Address Rule (Þágufall / Dative):

Foreldrar mínir eiga heima Hverfisgötu 12. (My parents live at Hverfisgata 12.)
Búðin er staðsett Skólavörðustíg 4. (The shop is located at Skólavörðustígur 4.)

Magn (Quantity, Age, and Size)

This is a common tripping point. Are you describing a state, or a difference?

Describing Measurement/Age (Eignarfall / Genitive): If you are stating how old someone is or how big something is, use the Genitive.

Húsið er fjögurra hæða. (The house is four stories high).

Barnið er tveggja mánaða. (The child is two months old.)
Fjallið er þúsund metra (hátt). (The mountain is a thousand meters high.)
Sundlaugin er fimmtíu metra (löng). (The swimming pool is fifty meters long.)
Ferðalagið er þriggja vikna (langt). (The trip is three weeks long.)

Describing Difference (Þágufall / Dative): If you are comparing two things and stating the difference between them, use the Dative.

Jón er fimm sentimetrum lægri en Anna. (Jón is five centimeters shorter than Anna.)
Taskan mín er þremur kílóum þyngri en þín. (My bag is three kilos heavier than yours.)
Þessi bíll er tveimur milljónum dýrari en hinn. (This car is two million [krónur] more expensive than the other one.)
Hann mætti tíu mínútum seinna en ég. (He arrived ten minutes later than I did.)