Hún útskýrði ástæðuna svo skýrt að allir skildu hana.

Here is a quick breakdown:

• Hún = subject pronoun, nominative singular, she
• útskýrði = past tense of útskýra, explained
• ástæðuna = direct object, accusative singular definite of ástæða, the reason
• svo skýrt = adverbial phrase, so clearly
• að = subordinating conjunction, that
• allir = subject of the subordinate clause, nominative plural, everyone / all people
• skildu = past tense plural of skilja, understood
• hana = accusative singular feminine pronoun, referring back to ástæðuna

Because it is the direct object of útskýra.

The dictionary form is ástæða = reason. In this sentence, it means the reason, and it is in the accusative singular because útskýra takes a direct object.

A useful mini-pattern is:

• ástæða = a reason
• ástæðan = the reason
• ástæðu = a reason, accusative
• ástæðuna = the reason, accusative

So ástæðuna means the reason in the object form required by the verb.

Because skýrt is being used adverbially here.

The adjective is skýr = clear. But in this sentence, the word describes how she explained, not what kind of reason it was. So the meaning is clearly, not clear.

In Icelandic, many adverbs have the same form as the neuter singular adjective, so:

• skýr = clear
• skýrt = clearly

Compare:

• Hún útskýrði það skýrt = She explained it clearly.
• skýra ástæðu = a clear reason

So here skýrt modifies the verb útskýrði.

It means so ... that.

This is a very common Icelandic pattern for showing degree plus result:

• svo skýrt að ... = so clearly that ...
• svo hratt að ... = so fast that ...
• svo vel að ... = so well that ...

So the sentence structure is:

• she explained the reason so clearly
• that everyone understood it

The að clause gives the result.

Yes, but the meaning changes.

• mjög skýrt = very clearly
• svo skýrt að ... = so clearly that ...

If you want the result clause að allir skildu hana, then svo is the natural choice.

So:

• Hún útskýrði ástæðuna mjög skýrt. = She explained the reason very clearly.
• Hún útskýrði ástæðuna svo skýrt að allir skildu hana. = She explained the reason so clearly that everyone understood it.

Because it is plural.

Allir literally means all or all people. Icelandic often expresses everyone with a plural form like this, where English uses a singular-looking word.

So allir skildu hana is literally something like all understood it, but naturally it means everyone understood it.

Because masculine plural is the default for a mixed group or an unspecified group of people.

So if you are talking about everyone in general, allir is normal.

Compare:

• allir = all, everyone, default/mixed/unspecified group
• allar = all, if the whole group is female

This does not mean the sentence is talking only about men. It is just standard Icelandic grammar.

Because its subject is allir, which is plural.

Icelandic verbs agree with the subject in person and number. Here the subject of the subordinate clause is allir, so the verb must also be plural:

• hann skildi = he understood
• hún skildi = she understood
• þau skildu = they understood
• allir skildu = everyone understood

So skildi would be singular, but skildu is the correct plural form here.

It refers to ástæðuna.

Even though English would normally say it, Icelandic uses a pronoun that matches the noun’s grammatical gender. Since ástæða is feminine, the pronoun is feminine too:

• ástæða = feminine noun
• hana = her/it, feminine accusative singular

So allir skildu hana means everyone understood it, where it = the reason.

Grammatically, hana could also mean her, but in this sentence the context clearly points back to ástæðuna.

Because skilja takes a direct object in the accusative.

The relevant pronoun forms are:

• hún = she
• hana = her / it, accusative
• henni = her, dative

Since skilja here means understand something, the thing understood is a direct object, so the accusative is needed:

• skildu hana = understood it
• not skildu henni

No. It is the same spelling, but not the same job.

Here að is a subordinating conjunction meaning that. It introduces the result clause:

• svo skýrt að allir skildu hana = so clearly that everyone understood it

This is different from infinitive að, as in:

• að skilja = to understand
• að útskýra = to explain

So in your sentence, að means that, not to.

Because after að, you have a subordinate clause, and the normal order here is:

• conjunction + subject + verb + object

So:

• að
  • allir
    • skildu
      • hana

In the main clause, Icelandic often follows the verb-second pattern. But in this sentence the subject is already first:

• Hún útskýrði ástæðuna ...

So it looks similar to English. The subordinate clause after að is also quite straightforward here.

They are:

• útskýrði → dictionary form útskýra = to explain
• skildu → dictionary form skilja = to understand

In this sentence:

• útskýrði is 3rd person singular past: she explained
• skildu is 3rd person plural past: they/everyone understood

So if you want to look them up, look for útskýra and skilja.