Learn Hardcore Icelandic: Hún undirbýr sig fyrir prófið í kvöld. - She prepares herself for the exam tonight.

Because undirbúa sig means “to prepare oneself.” The reflexive sig refers back to the subject (hún). Without sig, undirbúa would need some other direct object (e.g., “prepare the test” = undirbúa prófið).

Correct: Hún undirbýr sig … (She prepares herself …)
Different meaning: Hún undirbýr prófið … (She prepares the test …, e.g., a teacher)

Why can’t I use hana instead of sig?

Hana is the non-reflexive 3rd-person feminine accusative pronoun and refers to another woman, not the subject. To refer back to the subject, Icelandic requires the reflexive set (sig/sér/sín). So:

Hún undirbýr hana = She prepares her (someone else).
Hún undirbýr sig = She prepares herself.

What case is sig, and what are the other reflexive forms?

Sig is accusative. The reflexive pronouns (3rd person, all genders, singular and plural) are:

Accusative: sig
Dative: sér
Genitive: sín

Which one you use depends on the verb or preposition’s case requirements. Here, undirbúa sig takes accusative.

What does fyrir do here, and which case does it take?

Fyrir means “for” (in the sense of purpose or in preparation for) and takes the accusative in this meaning. Hence fyrir prófið (accusative). With other meanings (e.g., “in front of,” temporal “before”), the case can differ, but for “prepare for [something]” it’s accusative.

Why is it prófið and not just próf?

Icelandic marks the definite article as a suffix. Próf is “exam/test,” and prófið is “the exam/test.” The sentence talks about a specific exam, so the definite form is used.

Indefinite: próf
Definite: prófið

What are the basic forms of the noun próf?

Key forms:

Singular: nominative/accusative próf, dative prófi, genitive prófs
Definite singular: prófið
Plural: nominative/accusative próf, definite plural prófin

Note: Neuter nouns like próf have the same form in nominative and accusative.

Can I drop sig and say just Hún undirbýr fyrir prófið?

No. Undirbúa is transitive and needs a direct object. If you mean “prepare oneself,” you must include the reflexive: undirbúa sig. If you drop sig, you must supply another object (e.g., undirbýr prófið, undirbýr lestina “prepares the lecture,” etc.). Saying undirbúa fyrir without an object is ungrammatical.

Are there alternative, natural ways to say “prepare for the exam”?

Yes:

undirbúa sig fyrir próf(ið) (as in the sentence)
búa sig undir próf(ið) (very common)
undirbúa sig til prófs (idiomatic/formal; til takes genitive)

All are good. Nuance is minor; til prófs sounds a bit more formal or set-phrase-like.

Could í kvöld go in a different place? What about verb-second word order?

Yes. Icelandic main clauses are verb-second (V2). If you front the time phrase, the finite verb must remain in second position:

Neutral: Hún undirbýr sig fyrir prófið í kvöld.
Time-fronted: Í kvöld undirbýr hún sig fyrir prófið.

Both are natural. Fronting Í kvöld emphasizes “tonight.”

How do I negate this sentence?

Place ekki after the finite verb (and after short unstressed objects like sig):

Hún undirbýr sig ekki fyrir prófið í kvöld.

How do I turn it into a question?

Yes–no question (verb first): Undirbýr hún sig fyrir prófið í kvöld?
Wh-question: Hvenær undirbýr hún sig fyrir prófið? (“When does she prepare for the exam?”)

Does í kvöld mean “in the evening” or specifically “tonight”?

Í kvöld means “tonight” (this coming evening). It’s a fixed time expression and does not take an article. For habitual “in the evenings,” you’d use something like á kvöldin (“in the evenings,” habitually). A very specific “in the (named) evening” could use a definite form, but that’s uncommon in this sense.

Is there any ambiguity about whether the exam is tonight or the preparation is tonight?

Potentially, yes: í kvöld could attach to the preparing or to the exam. To disambiguate:

Preparation tonight (exam unspecified): Í kvöld undirbýr hún sig fyrir prófið.
Exam tonight (preparation at another time): Hún undirbýr sig í dag fyrir prófið í kvöld.

Can I use a progressive-like form?

Yes, with vera að + infinitive:

Hún er að undirbúa sig fyrir prófið. (She is preparing herself for the exam.) If you keep í kvöld, it usually means the action will take place tonight: Hún er að undirbúa sig fyrir prófið í kvöld can be interpreted as a plan for tonight unless context makes it literally current.

How is undirbýr conjugated, and what are some key forms of undirbúa?

Verb: undirbúa (to prepare)

Present: ég undirbý, þú undirbýrð, hann/hún/það undirbýr, við undirbúum, þið undirbúið, þeir/þær/þau undirbúa
Past: ég undirbjó, þú undirbjóst, hann/hún/það undirbjó, við undirbjuggum, þið undirbjugguð, þeir/þær/þau undirbjuggu
Past participle: undirbúinn/undirbúin/undirbúið (gender/number agreement)

Is there a middle-voice -st alternative instead of using sig?

For this meaning, Icelandic normally uses the reflexive: undirbúa sig. There is a form undirbúast, but it’s used more passively/impersonally (“to be under preparation”) rather than “prepare oneself.” So for a person getting ready, stick with undirbúa sig or búa sig undir.

Any quick pronunciation tips for the words in this sentence?

Stress is on the first syllable of each word: Hún, UNdirbýr, SIG, FYRir, PRÓfið, KVÖLD.
ú in Hún is like long “oo” (as in “food”).
ý in undirbýr sounds like long “ee” (same as í in modern Icelandic).
ó in prófið is like the “o” in “go.”
ö in kvöld is like the vowel in German “schön”; approximate with the “u” in “fur” but rounded.
ð in prófið is like the “th” in “this.”
Final -ld in kvöld is pronounced with a hard “t”-like ending ([lt]).

How would this sentence change with different subjects?

He: Hann undirbýr sig fyrir prófið í kvöld.
They (mixed/unspecified): Þau undirbúa sig fyrir prófið í kvöld.
Note the verb agreement: singular undirbýr, plural undirbúa. The reflexive sig stays the same for 3rd person regardless of gender/number.