Learn Hardcore Icelandic: Strákurinn er snjall í stærðfræði.

Here the adjective is predicative (after er “is”). With this adjective, the common predicative masculine form is snjall. Before a noun (attributive), you use snjallur: snjallur strákur (a smart boy). You may also hear Hann er snjallur, but in the “good at X” construction, snjall is very idiomatic.

How does the adjective agree with the subject?

Adjectives agree in gender and number.

Masculine: Hann/Strákurinn er snjall.
Feminine: Hún/Stúlkan er snjöll.
Neuter: Það/Barnið er snjallt.
Plural: Þeir/Þær/Þau/Krakkarnir eru snjallir/snjallar/snjöll (masc./fem./neut. plural).

Is snjall the most natural way to say “good at math”?

It’s fine and common, especially as “clever/skillful (in).” Other very common options:

góður í stærðfræði = good at math (neutral, very common)
klár í stærðfræði = clever/smart in math (colloquial)
sterkur í stærðfræði = strong in math (often used about school subjects) Nuance: snjall can sound a bit “clever/brilliant,” góður is the safest all‑purpose choice.

Why is it í and not á here?

For ability in a domain or subject, Icelandic uses í + dative: snjall/góður/klár í [subject]. So: snjall í stærðfræði. The preposition á is not used for this “good at (a subject)” pattern.

What case does í take here?

Dative, because it’s a static “in (a field/domain).” In general:

í + dative = in/inside (location or domain): í stærðfræði, í skólanum (in the school)
í + accusative = into/going to (direction): í skólann (to the school)

Can I say í stærðfræðinni? How would that change the meaning?

Yes. í stærðfræðinni (dative definite) points to a specific math class/course or a specific context (“in the math class,” “in that math course”). í stærðfræði is about the subject in general.

Is stærðfræði singular or plural? Does it decline?

stærðfræði (mathematics) behaves like a singular, uncountable feminine noun and is often unchanged across cases in everyday use:

að læra stærðfræði (to study math)
góður í stærðfræði (good at math) You’ll also see definite forms like stærðfræðin (the mathematics) and í stærðfræðinni (in the math class), but you generally won’t use a plural for the field.

How do I pronounce the tricky parts?

á = like the vowel in “now” (ow).
æ = like “eye.”
í = like “ee.”
ð = usually like “th” in “this.” In clusters (e.g., in stærðfræði), the first ð may sound more like the voiceless “th” in “thing” due to the following f. Approximate:
Strákurinn ≈ “STRAU-koo-rin” (stress on the first syllable).
snjall ≈ “snyall” (the “j” indicates a y-glide; one syllable).
stærðfræði ≈ “STIRETH-fraye-dhi” (first “th” voiceless, second voiced).

What is er?

er is the 3rd person singular present of vera (to be). Present tense:

ég er, þú ert, hann/hún/það er, við erum, þið eruð, þeir/þær/þau eru.

How do I make a yes/no question or a negation?

Yes/no question (verb-first): Er strákurinn snjall í stærðfræði?
Negation with ekki after the verb: Strákurinn er ekki snjall í stærðfræði.
You can add intensifiers: Strákurinn er mjög snjall í stærðfræði.

How do I say “the smart boy” versus “the boy is smart”?

Attributive before the noun (weak ending with definiteness): Snjalli strákurinn = the smart boy.
Predicative after “be” (strong form): Strákurinn er snjall = the boy is smart.

Could I replace Strákurinn with a pronoun?

Yes: Hann er snjall í stærðfræði. If the referent is a girl: Hún er snjöll í stærðfræði. For a child of unknown gender: Barnið er snjallt í stærðfræði.

Is strákur the only word for “boy”?

No. strákur is very common and neutral/colloquial. drengur is a bit more formal/literary. Both fit here:

Drengurinn er snjall í stærðfræði.