Hún er mjög skipulögð og ábyrg, svo hún setur alltaf það mikilvægasta í forgang.

Because the adjective has to agree with hún, which is feminine singular.

After vera in Icelandic, predicate adjectives still match the subject in gender and number:

• Hann er mjög skipulagður = masculine
• Hún er mjög skipulögð = feminine
• Það er mjög skipulagt = neuter

So skipulögð is the correct feminine singular form here.

For the same reason: it agrees with hún.

The adjective ábyrgur means responsible, and its singular nominative forms are:

• ábyrgur = masculine
• ábyrg = feminine
• ábyrgt = neuter

Since the subject is hún, you use the feminine form ábyrg.

Mjög means very. It is an adverb, and adverbs do not agree with gender or number the way adjectives do.

So it stays mjög no matter what:

• mjög skipulögð
• mjög ábyrg
• mjög gott

It is completely invariable.

Here svo means so in the sense of therefore / as a result.

So the sentence structure is:

• Hún er mjög skipulögð og ábyrg = she is very organized and responsible
• svo hún setur alltaf... = so she always puts...

Be aware that svo can mean other things in other contexts, such as so, then, or like that, but here it is clearly linking cause and result.

Because svo hún setur alltaf... is a new clause, and Icelandic normally states the subject explicitly in each clause.

English sometimes allows a more reduced style, but Icelandic usually prefers the full clause:

• Hún er mjög skipulögð og ábyrg, svo hún setur...

Repeating hún sounds natural and clear.

Setur is the 3rd person singular present tense of setja, which means to set / put.

So:

• ég set
• þú setur
• hún setur

In this sentence, the present tense is used for a habitual action, especially because of alltaf:

• hún setur alltaf... = she always puts / she always prioritizes

That is the normal word order in a main clause like this.

A neutral Icelandic pattern here is:

• subject + finite verb + adverb + object/prepositional phrase

So:

• hún = subject
• setur = finite verb
• alltaf = adverb
• það mikilvægasta = object
• í forgang = prepositional phrase

This is why hún setur alltaf... sounds natural, while hún alltaf setur... would not be normal standard word order here.

Because Icelandic is turning the adjective into a noun-like expression meaning the most important thing or what is most important.

The adjective is mikilvægur = important.
Its superlative is mikilvægastur = most important.

But when Icelandic uses a pronoun/article-like word such as það in front of it, the adjective takes the weak superlative form:

• mikilvægasta

So það mikilvægasta means something like:

• the most important thing
• what is most important

Because it is being used in an abstract, general way, as if an unstated neuter noun were behind it, such as hlut = thing or atriði = point/item.

So það mikilvægasta is basically:

• the most important thing

Neuter singular is very common in Icelandic for this kind of general, abstract reference.

Also, here it is the object of setur, but for neuter singular the nominative and accusative forms look the same, so you still see það mikilvægasta.

This is a very common learner question.

The phrase setja eitthvað í forgang means to prioritize something. Here í takes the accusative because the expression suggests putting something into a position or state of priority.

So:

• setja það í forgang = put it into priority = prioritize it

By contrast, í forgangi is used when something is already in that state:

• Það er í forgangi = it is being prioritized / it is high priority

So the contrast is roughly:

• setja í forgang = movement/change into priority
• vera í forgangi = being in priority

It is a normal and very common Icelandic expression meaning to prioritize.

Literally, it looks like put ... into priority, but in real usage it functions as the natural equivalent of prioritize.

So hún setur alltaf það mikilvægasta í forgang is a very idiomatic, natural sentence.

You may also encounter the verb forgangsraða, which also means to prioritize, but setja í forgang is extremely common and easy to understand.