Learn Hardcore Danish: Hun får rabat, fordi hun køber to par sko på udsalg. - She gets a discount because she buys two pairs of shoes on sale.

har (to have) → Hun har rabat would sound like She has a discount (as a permanent state), which is not what is meant.
bliver (to become / to get in the sense of to turn into) → Hun bliver rabat would literally be She becomes discount, which makes no sense.

So får is the normal verb when someone is given or gets something.

Why is rabat used without an article? Why not Hun får en rabat?

In Danish, rabat is a common-gender noun (en rabat), but in practice you nearly always say:

Hun får rabat (She gets a discount).

Many abstract or “mass-like” nouns in Danish are often used without an article when you talk about them in a general or typical way, especially in set expressions with få:

få hjælp (get help)
få løn (get paid / get salary)
få rabat (get a discount)

You can say en rabat in some contexts, but it usually feels like a particular, clearly delimited discount, e.g. en rabat på 10% (a 10% discount). In this sentence, the general, article‑less rabat is natural.

Why is there a comma before fordi? Is it required?

Danish has two accepted comma systems. In modern usage:

Putting a comma before fordi is optional but very common:
- Hun får rabat, fordi hun køber to par sko på udsalg.
You can also write it without the comma:
- Hun får rabat fordi hun køber to par sko på udsalg.

Both are correct according to current rules.
The important grammar point: fordi introduces a subordinate clause (a reason clause), and that affects word order after fordi (see next question).

Why is the word order hun køber after fordi, and not køber hun?

Danish distinguishes word order between main clauses and subordinate clauses:

Main clause (independent sentence): verb is in second position
- Hun får rabat. (Subject Hun, verb får in position 2)
Subordinate clause (introduced by fordi, at, når, som, etc.): the subject comes before the verb
- fordi hun køber to par sko på udsalg
  - Subject: hun
  - Verb: køber

So:

Main clause style Får hun rabat? (for a yes/no question)
Subordinate clause style … fordi hun får rabat

Køber hun after fordi would sound like a question and is ungrammatical as a normal subordinate clause.

Why do we repeat hun in the second part? Could we say Hun får rabat, fordi køber to par sko…?

You must repeat the subject pronoun in Danish. Every finite clause needs its own explicit subject (unless it is an imperative).

So the second clause needs hun:

… fordi hun køber to par sko på udsalg.

Leaving it out (… fordi køber to par sko …) is ungrammatical.

Unlike some languages, Danish does not generally allow “null subjects” (dropping the pronoun) in finite clauses.

Why is the tense present (får, køber) if this can describe a future situation?

Danish uses the present tense much more than English does, including for future meaning.

Hun får rabat, fordi hun køber to par sko på udsalg.
- Can mean: She gets a discount because she buys two pairs of shoes on sale
- In context, it can also correspond to English will:
  - She will get a discount because she is buying two pairs of shoes on sale.

If you want to be very explicitly future, you can say:

Hun vil få rabat, fordi hun køber to par sko på udsalg.

But in everyday Danish, plain present is usually enough when the future time is clear from context.

What exactly does to par sko mean, and why isn’t it to parrer sko or something like that?

To par sko literally means two pairs of shoes. The structure is:

to = two
par = pair (here used as a count word / measure word)
sko = shoes

In this kind of construction, par itself normally stays singular after a number:

et par sko (one pair of shoes)
to par sko (two pairs of shoes)
tre par sko (three pairs of shoes)

You do not say parrer in this meaning. Parrer is the verb at parre (to mate animals, to pair things), not the plural of par. The plural of par is also par, but it is not used in this numeric construction; we just say to par sko, tre par strømper, etc.

Why does sko look the same in singular and plural? How do I know it’s plural here?

Sko is an irregular noun:

Singular: en sko (a shoe)
Plural: sko (shoes)

So the form is the same for singular and plural; you know it’s plural here because of the number word and par:

et par sko = one pair → 2 shoes
to par sko = two pairs → 4 shoes

Context and the presence of par and to tell you you’re talking about multiple shoes.

What does på udsalg literally mean, and is it the normal way to say on sale?

Yes, på udsalg is the natural, idiomatic way to say on sale in the sense of at a reduced price.

udsalg = sale (as in a sale event, clearance sale, reduced prices)
på udsalg = on sale / in a sale

So sko på udsalg = shoes on sale.

You do not normally say i udsalg in this sense. På udsalg is the fixed expression.

Why is udsalg also without an article? Why not på et udsalg?

Udsalg is a common-gender noun (et udsalg in the indefinite form), but in the phrase på udsalg it works almost like an adverbial state:

på arbejde (at work)
på ferie (on holiday)
på udsalg (on sale)

In these fixed expressions, Danish typically drops the article.

You can say på et udsalg when you really mean at a (particular) sale event, like at a certain store, but sko på udsalg in this sentence just means shoes that are on sale (reduced price) in general.

Could you replace fordi with når or hvis? Would it change the meaning?

Yes, and it would change the nuance:

fordi = because, expresses a reason that is (or is seen as) real or factual
- Hun får rabat, fordi hun køber to par sko på udsalg.
  - She gets a discount because she buys two pairs of shoes on sale.
når = when / whenever (repeated or general situations)
- Hun får rabat, når hun køber to par sko på udsalg.
  - She gets a discount whenever she buys two pairs of shoes on sale.
hvis = if (condition)
- Hun får rabat, hvis hun køber to par sko på udsalg.
  - She gets a discount if she buys two pairs of shoes on sale.

So fordi is specifically about cause/reason, not a condition.

Can på udsalg be moved, for example Hun får rabat, fordi hun på udsalg køber to par sko?

In theory, Danish word order is quite flexible, but naturalness matters.

Hun får rabat, fordi hun køber to par sko på udsalg.
- This is the most natural, idiomatic version.

You could say:

Hun får rabat, fordi hun køber to par sko, der er på udsalg.
- … because she buys two pairs of shoes that are on sale.

Putting på udsalg between hun and køber (hun på udsalg køber) would sound very strange and unidiomatic. Adverbials like på udsalg usually come later in the clause, after the verb and object, unless there is a strong emphasis or very specific stylistic reason.