Eldingin var svo björt að barnið varð hrætt.

Those endings are the suffixed definite article in Icelandic. Instead of putting a separate word like the before the noun, Icelandic often attaches it to the end.

• elding = lightning

• eldingin = the lightning

• barn = child
• barnið = the child

In this sentence, both nouns are definite.

Because grammatical gender and natural gender are not the same thing.

In Icelandic, barn is simply a neuter noun, no matter whether the child is a boy or a girl. That affects the forms of words that agree with it, such as the article and adjectives.

So:

• barn is neuter
• barnið is therefore neuter
• the adjective later becomes hrætt, which is also neuter singular

This does not mean the child is being called genderless in a real-world sense; it is just grammar.

Because adjectives in Icelandic agree with the noun they describe.

The dictionary form is usually the masculine nominative singular, so you often learn the adjective as bjartur. But here it describes eldingin, which is feminine singular nominative, so the adjective changes form:

• masculine: bjartur
• feminine: björt
• neuter: bjart

Since eldingin is feminine, the sentence uses björt.

Also, this is a predicate adjective after var, not an adjective directly before the noun, but it still agrees the same way.

For the same reason: agreement.

The adjective hræddur means afraid, but hræddur is just the masculine nominative singular form. In the sentence, the adjective describes barnið, and barnið is neuter singular, so the adjective must also be neuter singular:

• masculine: hræddur
• feminine: hrædd
• neuter: hrætt

So barnið varð hrætt is grammatically correct because both the noun and the adjective are neuter singular.

They come from two different verbs:

• var = past tense of vera = was
• varð = past tense of verða = became, got

So:

• Eldingin var svo björt = The lightning was so bright...
• barnið varð hrætt = ...the child became/got afraid

This distinction matters. If you said barnið var hrætt, that would mean the child was afraid, describing a state.
But barnið varð hrætt shows a change into that state.

It is a very common Icelandic pattern meaning so ... that.

In this sentence:

• svo björt = so bright
• að barnið varð hrætt = that the child became afraid

So svo ... að introduces a result:

so bright that the child became afraid

You can think of it as a set phrase or structure:

• svo stór að ... = so big that ...
• svo kalt að ... = so cold that ...
• svo erfitt að ... = so difficult that ...

Here, að is a subordinating conjunction meaning that.

It introduces the clause barnið varð hrætt.

That is different from other uses of að, for example:

• að lesa = to read
  Here að is the infinitive marker, like English to.

But in your sentence, it is not to. It is the that in so bright that...

Because after a subordinating conjunction like að, Icelandic normally uses regular clause order, with the subject before the finite verb, unless some other element is deliberately placed first.

So:

• að barnið varð hrætt = that the child became afraid

This is normal subordinate-clause order.

Learners often notice that Icelandic main clauses often have verb-second behavior, but subordinate clauses introduced by words like að are usually more straightforward in their order.

They are in the nominative.

Why?

• eldingin is the subject of the first clause
• barnið is the subject of the second clause

The predicate adjectives björt and hrætt agree with those subjects, so they also appear in nominative forms.

So the basic pattern is:

• subject in nominative
• vera/verða
• predicate adjective agreeing with the subject

Because article use does not match perfectly between Icelandic and English.

Icelandic often uses the definite form when talking about a specific instance of something already present in the situation, such as a particular flash of lightning. English may prefer a bare noun in the same context.

So even if English says simply lightning, Icelandic may naturally say eldingin.

This is something learners usually have to get used to: Icelandic and English do not always make the same article choice.

In this sentence, it most naturally refers to a specific flash or instance of lightning.

That fits well with:

• the definite form eldingin
• the result clause that the child became afraid

So the image is not really lightning as a general phenomenon, but rather the lightning flash that was so bright that it scared the child.