Ástæðan er enn ekki skýr.

Because -n is the suffixed definite article. So:

• ástæða = reason
• ástæðan = the reason

Icelandic usually adds the to the end of the noun instead of using a separate word like English does.

It is nominative singular. It is the subject of the sentence, so nominative is the expected case.

A useful contrast:

• ástæðan = nominative, the reason
• ástæðuna = accusative, the reason as a direct object

er is the present-tense singular form of vera, which means to be.

So the structure is simply:

• Ástæðan = the reason
• er = is
• enn ekki skýr = still not clear

Because Icelandic main clauses normally follow a verb-second pattern. That means the finite verb usually comes second in the clause.

Here, the first element is Ástæðan, so the verb er comes next:

• Ástæðan | er | enn ekki skýr

This is a very common feature of Icelandic word order.

Because skýr is being used predicatively, not attributively.

In other words, the sentence is built like:

• The reason is clear

not like:

• the clear reason

So:

• Ástæðan er skýr = The reason is clear
• skýr ástæða = a clear reason

The adjective agrees with the subject in gender, number, and case.

Ástæða is feminine singular nominative, so the adjective must match that. With skýr, the nominative singular masculine and feminine forms happen to look the same:

• masculine: skýr
• feminine: skýr
• neuter: skýrt

So skýrt would be wrong here, because the subject is not neuter.

Also, after er, Icelandic uses the predicative adjective form, not a weak attributive form like skýra.

Here enn means still.

So enn ekki gives the sense of still not. Depending on context, English might sometimes translate this kind of idea as not yet, but in this sentence still not is the most direct match.

In a normal Icelandic main clause, ekki usually comes after the finite verb.

So without enn, you would get:

• Ástæðan er ekki skýr = The reason is not clear

When enn is added, it commonly appears before ekki:

• Ástæðan er enn ekki skýr

So the sequence er enn ekki is a natural way to say is still not.

Yes. Ástæða often means reason, but in some contexts it can also be translated as cause or explanation.

So depending on what the wider context is, English might choose:

• The reason is still not clear
• The cause is still not clear

Both can fit ástæðan.

Yes, but the sentence may sound more marked or shift the emphasis. Because Icelandic is verb-second, the finite verb still usually stays in second position.

For example:

• Enn er ástæðan ekki skýr.

This is possible and gives more prominence to enn.

The version Ástæðan er enn ekki skýr is a neutral, natural order.

A few helpful sound notes:

• á is roughly like ow in now
• æ is roughly like eye
• ð is like th in this
• ý is roughly like ee in see
• stress in Icelandic is usually on the first syllable

So very roughly:

• Ástæðan ≈ OW-stai-than
• skýr has a long ee-like vowel

The exact Icelandic pronunciation is a bit more precise than these English approximations, but those hints will get you close enough to recognize the sentence.