Vitnið mun útskýra afleiðinguna á vettvanginum.

The basic noun is vitni, meaning witness. The ending -ið makes it definite, so vitnið means the witness.

Icelandic usually puts the onto the end of the noun instead of using a separate word. So:

• vitni = a witness / witness
• vitnið = the witness

In this sentence, vitnið is also the subject of the verb.

Because grammatical gender in Icelandic does not always match natural gender.

The noun vitni is simply a neuter noun in Icelandic, even though a witness is of course a person. That is normal: some Icelandic nouns referring to people are grammatically neuter.

So in this sentence, the grammar treats vitni as neuter, but that does not mean the witness is not male or female. It only tells you the noun’s grammatical class.

Mun is the verb form used here to express the future.

It comes from the verb munu, and in this sentence mun is the 3rd person singular form, matching vitnið.

So:

• vitnið mun útskýra = the witness will explain

This is a common way to make a future meaning in Icelandic, although Icelandic can also often use the present tense for future meaning depending on context.

Because after mun, the main verb stays in the infinitive.

That is very similar to English:

• will explain
• mun útskýra

You do not use að here. So it is:

• mun útskýra

not:

• mun að útskýra

Because afleiðinguna is the direct object of útskýra, and útskýra takes the accusative case.

The noun is:

• afleiðing = consequence, result

Its forms include:

• afleiðing = nominative singular indefinite
• afleiðingin = nominative singular definite
• afleiðinguna = accusative singular definite

Since the witness is explaining the consequence, Icelandic uses the accusative definite form:

• útskýra afleiðinguna

So afleiðinguna means the consequence as the object of the verb.

Because the preposition á can take different cases depending on the meaning.

With location (something happening at/on a place), á takes the dative.
With motion toward a place, á takes the accusative.

Here the sentence is talking about where the explaining happens, so it is location, not movement. That is why Icelandic uses:

• á vettvanginum = at the scene / on the scene

If the meaning were something like onto the scene or to the scene, you would expect the accusative form instead.

Vettvangur can mean site, scene, venue, arena, or field, depending on context.

In a sentence like this, á vettvanginum most naturally means:

• at the scene
• on the scene
• at the site

In news, police, or accident-related language, vettvangur very often refers to the scene of an event.

Yes. This is normal Icelandic main-clause word order.

A very important rule in Icelandic is that the finite verb tends to be in second position in main clauses. Here:

• Vitnið = first element
• mun = finite verb in second position
• útskýra = infinitive later in the clause

So the structure is perfectly normal:

• Vitnið mun útskýra afleiðinguna á vettvanginum.

If you moved another element to the front, the finite verb would still stay in second position, for example:

• Á vettvanginum mun vitnið útskýra afleiðinguna.

That verb-second pattern is a key feature of Icelandic word order.