Learn Hardcore Icelandic: Hún þarf meiri hreyfingu, þar sem hún situr mikið við skrifborð. - She needs more exercise, since she sits a lot at a desk.

Hreyfing is a feminine noun meaning movement, exercise. In this sentence it’s the direct object of þarf, so it must be in the accusative singular:

Nominative: hreyfing (e.g. Hreyfing er holl. – Exercise is healthy.)
Accusative: hreyfingu (e.g. Ég þarf hreyfingu. – I need exercise.)

The verb að þurfa normally takes an object in the accusative, which is why you see hreyfingu.

Why is it meiri hreyfingu and not meira hreyfingu?

Meiri is the comparative form of the adjective mikill (much, many), and here it must agree with the noun hreyfingu in gender, number, and case:

hreyfingu: feminine, singular, accusative
Matching comparative: meiri (fem. sg. acc.)

Meira is the neuter or adverbial form and would usually be used:

on its own: Hún þarf meira. – She needs more.
as an adverb: Hún þarf að sofa meira. – She needs to sleep more.

Because meiri comes before a specific feminine noun (hreyfingu), meiri hreyfingu is the correct form.

What does þar sem mean here, and how is it different from af því að?

In this sentence þar sem means “since / because”:

Hún þarf meiri hreyfingu, þar sem hún situr mikið við skrifborð.
→ She needs more exercise, since/because she sits a lot at a desk.

þar sem can mean two things in Icelandic:

“where” (location):
- Hann býr þar sem ég ólst upp. – He lives where I grew up.
“since/because” (cause), as in your sentence.

af því að also means “because”, and is very common in speech:

Hún þarf meiri hreyfingu, af því að hún situr mikið við skrifborð.

The difference is mostly style/feel:

þar sem often sounds a bit more formal or written.
af því að is very common in everyday spoken language.

Why is there a comma before þar sem? Do I always put a comma before it?

Yes, you normally put a comma before a subordinate clause, and þar sem introduces such a clause.

Main clause: Hún þarf meiri hreyfingu,
Subordinate clause: þar sem hún situr mikið við skrifborð.

In standard Icelandic punctuation, subordinating conjunctions such as þar sem, af því að, þegar, ef, þó að etc. usually follow a comma when they start a subordinate clause after a main clause.

Why is hún repeated? Could I just say …, þar sem situr mikið við skrifborð?

You must repeat the subject pronoun here. Icelandic does not generally allow dropping the subject like that.

So:

Correct: …, þar sem hún situr mikið við skrifborð.
Incorrect / ungrammatical: …, þar sem situr mikið við skrifborð.

Each finite clause needs its own (usually explicit) subject:

Hún þarf… / hún situr…

What does mikið do here? Is it an adjective or something else?

In this sentence, mikið is used adverbially and means “a lot” / “a great deal”:

Hún situr mikið við skrifborð.
→ She sits a lot at a desk.

Even though mikill is originally an adjective (much, big), the neuter form mikið is often used as an adverb:

Hann vinnur mikið. – He works a lot.
Ég les mikið. – I read a lot.

Why is it við skrifborð and not á skrifborði? What does við mean here?

The preposition við usually means “by, next to, at”. With desks and tables, sitja við X is the normal, idiomatic way to say “sit at X”:

Ég sit við borð. – I sit at a table.
Hún situr við skrifborð. – She sits at a desk.

Við takes the accusative, and skrifborð is a neuter noun whose nominative and accusative singular forms are identical, so you don’t see a change in form.

If you said á skrifborði, it would more literally mean “on top of a desk”, suggesting she is physically sitting on the desk surface, not at it.

Why is there no the in Icelandic? Why not skrifborðinu for “the desk”?

Icelandic usually attaches the definite article as a suffix to the noun:

skrifborð – (a) desk
skrifborðið / skrifborðinu – the desk (different cases)

In your sentence:

við skrifborð → at a desk / at a (her) desk in a general sense.

If you said:

við skrifborðið / við skrifborðinu

you’d be referring to a specific, known desk (at the desk), which slightly changes the nuance. The generic idea “she sits a lot at a desk (i.e. has a desk job)” fits better with við skrifborð.

Could I also say Hún þarf að hreyfa sig meira instead of Hún þarf meiri hreyfingu?

Yes, both are natural, but they differ slightly in structure:

Hún þarf meiri hreyfingu.
- Literally: She needs more exercise.
- Uses the noun hreyfing (exercise) in the accusative.
Hún þarf að hreyfa sig meira.
- Literally: She needs to move herself more.
- Uses the verb phrase að hreyfa sig (to move, to exercise) and the adverb meira.

Meaning-wise they are very close. The first focuses a bit more on “exercise” as a thing, the second on the action “to move more”. Both are perfectly idiomatic.

Can I move the þar sem-clause to the beginning of the sentence?

Yes, that’s possible and correct:

Þar sem hún situr mikið við skrifborð, þarf hún meiri hreyfingu.

This is just a different word order. Two things to notice:

You still keep a comma after the subordinate clause.
The verb in the main clause (þarf) must still be in second position after the subject hún, which you did: … þarf hún ….

Both orders are normal; the original version is probably more common in everyday style.

How do you pronounce some of the tricky words like þarf, hreyfingu, and skrifborð?

Very roughly, using English sounds (not perfect, but close enough to guide you):

þarf ≈ tharf
- þ like the th in think (never like this).
- Short a (like in fun, but a bit more open).
- Final rf often sounds close to rp/f in fast speech.
hreyfingu ≈ HRAY-ving-oo
- hr: a breathed r, a bit like saying h
  - r together.
- ey: like English “ay” in day.
- Stress on the first syllable: HREY-ving-oo.
skrifborð ≈ SKRIF-borth
- skr pronounced together, like skr in screw.
- i short, like i in bit.
- ð at the end is like a soft th in this, but often very weak or almost silent in fast speech.