Learn Hardcore Icelandic: Hún selur bækur á markaðnum um helgar. - She sells books at the market on weekends.

Because the verb selja takes an accusative direct object, and here we mean “books” (plural). Bækur is the nominative/accusative plural of bók “book.”

Singular: bók (nom/acc/dat), bókar (gen)
Plural: bækur (nom/acc), bókum (dat), bóka (gen) If you wanted “the books,” you’d say bækurnar (accusative plural definite). “Some books” can be nokkrar bækur.

What cases are used in the sentence, and why?

Hún is nominative (subject).
bækur is accusative (direct object of selur).
á markaðnum is dative because á takes the dative for location (“at/on” a place).
um helgar is accusative plural because um takes the accusative in time expressions (“during/on” weekends).

Why is it á markaðnum and not á markaðinn?

Use á + dative for location (being at/on a place) and á + accusative for direction (movement to/onto a place).

Location: Hún selur bækur á markaðnum. (She sells books at the market.)
Direction: Hún fer á markaðinn. (She goes to the market.) Indefinite “at a market” would be á markaði (dative, no article).

Where is the definite article “the”? Why no article before bækur?

Icelandic puts the definite article at the end of the noun. Markaðnum means “the market” (dative singular definite). There is no separate word like English “the.” Icelandic has no indefinite article (“a/an”), so plain plural bækur already means “books.” To make it definite: bækurnar (“the books”).

Can I say í markaðnum instead of á markaðnum?

Generally no; á markaðnum is the idiomatic way to say “at the market” (an event/place you go to). Í is more “in/inside,” used for enclosed spaces (e.g., í búðinni “in the shop”). You might use í with a specific indoor market building (e.g., í markaðshöllinni), but for “at the market” as an event, stick with á.

What exactly does um helgar mean? Why plural?

Um helgar means “on weekends/in the weekends” in a habitual, recurring sense. Um takes the accusative, and helgar is the accusative plural of helgi (“weekend”). For a specific weekend, use singular: um helgina (“over the weekend/this weekend”). For the coming one: um næstu helgi.

Can I move the time phrase around? What happens to word order?

Yes. Icelandic main clauses are verb-second (V2), so the finite verb stays in position 2 no matter what you front:

Hún selur bækur á markaðnum um helgar.
Um helgar selur hún bækur á markaðnum.
Á markaðnum selur hún bækur um helgar. All are fine; choose what you want to emphasize.

Why selur and not selar? How is selja conjugated?

Selur is the standard 2nd/3rd person singular present ending for many verbs like selja. Present of selja:

ég sel
þú selur
hann/hún selur
við seljum
þið seljið
þeir/þær/þau selja Preterite: seldi (ég seldi), past participle: selt.

How do I say “She is selling books …” (progressive) in Icelandic?

Use vera að + infinitive: Hún er að selja bækur … This stresses an action in progress right now. For general/habitual statements, the simple present (Hún selur …) is preferred.

How would I negate the sentence?

Place ekki after the finite verb:

Hún selur ekki bækur á markaðnum um helgar. For “never,” use aldrei: Hún selur aldrei bækur á markaðnum um helgar.

Could I talk about multiple markets (in markets)?

Yes. Dative plural is á mörkuðum (“at/in markets”). Definite dative plural is á mörkuðunum (“at the markets”):

Hún selur bækur á mörkuðum um helgar.
Hún selur bækur á mörkuðunum um helgar.

Any quick pronunciation tips for the tricky bits?

á sounds like the “ow” in “cow.”
æ (in bækur) sounds like “eye.”
Final -ur (as in selur, bækur) is a short rounded vowel plus r; think a quick “-ur.”
ð (in markaðnum) is like the soft th in “this.”
u in um is short and rounded (not like English “oo”).
Icelandic r is tapped or trilled.