Questions & Answers about Hann er mjög góður í stærðfræði.
Why does the sentence use í when the English translation says "at"?
In Icelandic, the fixed phrase for being "good at" something is vera góður í (literally "to be good in"). Prepositions rarely map perfectly between languages, so it is best to memorize góður í as a set phrase.
What case does the preposition í trigger for stærðfræði in this context?
It triggers the dative case. The preposition í can take either the accusative (usually for movement or direction) or the dative (for a static position or state). Since being good at math is a state, it takes the dative. The word stærðfræði is feminine and happens to look the same in all four cases in the singular, but it is grammatically dative here.
Why is the adjective góður used instead of gott?
Adjectives in Icelandic must agree with the noun or pronoun they describe in gender, number, and case. Because the subject hann (he) is masculine, singular, and in the nominative case, the adjective must be the masculine singular nominative form góður. If the sentence was about a woman, it would be Hún er mjög góð í stærðfræði.
How does a word like stærðfræði mean mathematics?
Icelandic often creates its own words for international concepts by combining native words. Stærð means "size" or "magnitude," and fræði means "science" or "the study of." Put them together, and math is literally the "study of sizes."