Þruman er svo hávær að hundurinn verður hræddur.

It is the Icelandic pattern for so ... that.

• svo hávær = so loud
• að hundurinn verður hræddur = that the dog gets scared

So svo intensifies the adjective, and að introduces the result clause.

A good way to read the structure is:

• Þruman er svo hávær = The thunder is so loud
• að hundurinn verður hræddur = that the dog gets scared

Because Icelandic usually puts the at the end of the noun as a suffix.

• þruma = thunder
• þruman = the thunder

With many feminine nouns ending in a vowel, the definite form in the nominative singular adds -n.

So Þruman is not a different word from þruma; it is the definite form.

Because hávær is describing Þruman, and adjectives in Icelandic agree with the noun they describe.

Here, Þruman is feminine singular, but the nominative singular form of hávær happens to look the same in both masculine and feminine:

• masculine: hávær
• feminine: hávær
• neuter: hávært

So the adjective is agreeing correctly; it just does not show a visible feminine ending in this form.

Also, after a verb like er (is), Icelandic often uses the adjective as a predicate adjective, just like English does in the thunder is loud.

Because the two verbs mean different things:

• er = is
• verður = becomes / gets

So:

• Þruman er svo hávær = The thunder is so loud
• hundurinn verður hræddur = the dog gets / becomes scared

This is a very common distinction in Icelandic:

• vera hræddur = to be scared
• verða hræddur = to become/get scared

So verður shows a change of state: the dog was not scared before, and then becomes scared.

For the same reason as Þruman: Icelandic often adds the definite article to the end of the noun.

• hundur = dog
• hundurinn = the dog

Here the sentence refers to a specific dog, so the definite form is used.

It is also the subject of the second clause, so it appears in the nominative case.

Because hræddur has to agree with hundurinn.

Hundurinn is:

• masculine
• singular
• nominative

So the adjective must match that:

• masculine singular nominative: hræddur

Other forms would be different, for example:

• feminine singular nominative: hrædd
• neuter singular nominative: hrætt

So if the subject were a feminine noun, you would not use hræddur.

Because this is a subordinate clause introduced by að.

In main clauses, Icelandic often follows a verb-second pattern. But after a conjunction like að, the subordinate clause usually has more straightforward subject-verb order:

• að hundurinn verður hræddur

So this is normal subordinate-clause word order:

• conjunction
• subject
• finite verb
• complement

For an English speaker, this often feels more familiar than main-clause Icelandic word order.

It is the same spelling, but not the same function.

Here, að is a conjunction meaning that:

• svo hávær að hundurinn verður hræddur
• so loud that the dog gets scared

But Icelandic also uses að as an infinitive marker meaning to:

• að verða hræddur = to become scared

You can tell which one it is from the structure:

• if að is followed by a full clause with a subject and a finite verb, it means that
• if að is followed by an infinitive, it means to

Both are in the nominative case, because both are subjects.

• Þruman is the subject of er
• hundurinn is the subject of verður

Even though the second subject is inside a subordinate clause, it is still the subject of that clause, so it stays nominative.

A few useful pronunciation points:

• Þ is like the th in thing
• ð is like the th in this, though at the end of a word it is often very soft
• æ is pronounced roughly like eye

So:

• Þruman starts with that thing-type th
• hávær contains æ, which sounds roughly like eye
• að has a soft ð at the end

Also, Icelandic usually stresses the first syllable of the word.

So even if your pronunciation is not perfect, getting the first-syllable stress right will already make you sound more natural.