Við setjum ferskar kryddjurtir á súpuna áður en við borðum.

Setja is the infinitive, meaning to put or to place.
In the sentence, setjum is the present-tense form for we:

• ég set = I put
• þú setur = you put
• við setjum = we put

So Við setjum ... means We put ... or We are putting ..., depending on context.

Because the sentence has two clauses:

1. Við setjum ferskar kryddjurtir á súpuna
2. áður en við borðum

Each clause needs its own subject, so við appears in both:

• við setjum = we put
• við borðum = we eat

Unlike some languages, Icelandic usually does not drop subject pronouns just because the verb ending already shows the person.

Because adjectives in Icelandic must agree with the noun they describe in:

• gender
• number
• case

Here, kryddjurtir is:

• feminine
• plural
• accusative (direct object)

So ferskur changes to ferskar to match it.

A useful way to think of it is:

• dictionary form: ferskur = fresh
• matching form here: ferskar = fresh, agreeing with feminine plural kryddjurtir

Because Icelandic very often makes compound nouns.

Kryddjurtir is built from:

• krydd = spice, seasoning
• jurt = herb, plant

So kryddjurt literally means something like seasoning-herb, and kryddjurtir is the plural: herbs.

This is very normal in Icelandic, and learning to spot compounds is extremely helpful.

Súpa means soup.
Súpuna means the soup in the accusative singular.

Icelandic usually puts the definite article on the end of the noun instead of using a separate word like the.

So:

• súpa = soup
• súpan = the soup, nominative
• súpuna = the soup, accusative

Here it is súpuna because the preposition á is taking the accusative in this sentence.

Because with some Icelandic prepositions, the case changes depending on whether the phrase shows:

• movement/direction
• or location

With á:

• á + accusative often shows movement onto something
• á + dative often shows being on something

Here, the herbs are being put onto the soup, so the phrase is directional:

• á súpuna

If you were describing where something already is, you would more likely use dative:

• Kryddjurtirnar eru á súpunni = The herbs are on the soup

Yes, in this sentence it means on the soup, as in adding herbs as a topping or garnish.

That makes sense because the herbs are being placed on top right before eating.

If you meant putting something into the soup more as an ingredient, Icelandic might use í instead:

• í súpuna = into the soup

So the choice between á and í depends on the image:

• á súpuna = on top of the soup
• í súpuna = into the soup

Áður en means before when it introduces a full clause.

So:

• áður en við borðum = before we eat

You can think of it as a fixed expression here:

• áður = before, earlier
• en = introducing the comparison/subordinate clause

Together they function as before in English.

Borðum is the present-tense we form of að borða = to eat.

So:

• ég borða = I eat
• þú borðar = you eat
• við borðum = we eat

In this sentence, við borðum simply means we eat.

It is indefinite: fresh herbs.

There is no definite ending on kryddjurtir, so it does not mean the fresh herbs.

Compare:

• ferskar kryddjurtir = fresh herbs
• fersku kryddjurtirnar = the fresh herbs

So the sentence is talking about herbs in a general sense, not a specifically identified set of herbs.

Yes. Icelandic word order is somewhat flexible, but the finite verb in a main clause normally stays in the second position.

The sentence as given is neutral and natural:

• Við setjum ferskar kryddjurtir á súpuna áður en við borðum.

You can also front the time clause:

• Áður en við borðum, setjum við ferskar kryddjurtir á súpuna.

Notice what happens in the main clause after fronting:

• setjum við
  not við setjum

That is a very important Icelandic pattern.