Það er andstæðingurinn sem kastar teningnum.

This pattern is called a cleft sentence. It is used to put special emphasis on one part of the sentence.

• Andstæðingurinn kastar teningnum = a neutral statement
• Það er andstæðingurinn sem kastar teningnum = It is the opponent who throws the die

So the point is not a different basic meaning, but a different focus. The speaker is highlighting andstæðingurinn.

This is very common in Icelandic, just as English uses It is X who/that ....

Not really in the normal referential sense.

In this sentence, það is a dummy subject or expletive. It does not refer to a thing. It is just part of the fixed cleft structure:

• Það er X sem ...

So here það is more structural than meaningful. English works the same way in It is the opponent who throws the die.

Because andstæðingurinn is the definite form: the opponent.

Breakdown:

• andstæðingur = an opponent
• andstæðingurinn = the opponent

Icelandic usually adds the definite article to the end of the noun rather than using a separate word like English the.

So the ending -inn here is the definite article, and the full form is nominative singular masculine.

Sem introduces the clause that describes or identifies the focused element.

In this sentence:

• andstæðingurinn = the focused person
• sem kastar teningnum = who throws the die

So sem is functioning like English who or that in this kind of sentence.

A useful way to think of it is:

• Það er andstæðingurinn sem kastar teningnum
• It is the opponent who throws the die

Because andstæðingurinn is already the understood subject of the clause after sem.

In other words:

• sem kastar teningnum = who throws the die

Icelandic does not add another subject pronoun there. You do not say something like sem hann kastar teningnum if andstæðingurinn is already the one doing the action.

So the relative clause has a kind of gap where the subject belongs, and that gap refers back to andstæðingurinn.

Kastar is the 3rd person singular present tense of kasta.

The infinitive is:

• að kasta = to throw

Present-tense forms include:

• ég kasta = I throw
• þú kastar = you throw
• hann/hún/það kastar = he/she/it throws

Since andstæðingurinn is singular, the verb is singular too:

• andstæðingurinn kastar

So kastar matches the opponent.

Because kasta normally takes a dative object in Icelandic.

So with að kasta:

• teningur = a die, nominative
• teningi = a die, dative indefinite
• teningnum = the die, dative definite

That means:

• kasta teningi = throw a die
• kasta teningnum = throw the die

This is something English speakers often have to memorize: some Icelandic verbs require a case that does not match English expectations.

A very useful note to remember is:

• að kasta einhverju = to throw something
  The einhverju form shows that the verb wants the dative.

Because Icelandic is clearly using the singular noun here.

• teningur = die
• teningar = dice

English often uses dice loosely even when only one die is meant, especially in casual speech. But Icelandic keeps the singular/plural distinction clearly here.

So teningnum means the die, not the dice.

It is nominative singular.

Even though the sentence begins with Það er, the noun after er in this kind of structure is normally in the nominative:

• Það er andstæðingurinn ...

This also makes sense because andstæðingurinn is the person identified as the one who performs the action in the clause after sem.

So the important contrast is:

• andstæðingurinn = nominative
• teningnum = dative

Yes. The simpler version is:

• Andstæðingurinn kastar teningnum.

That is a perfectly normal sentence.

The difference is mainly emphasis:

• Andstæðingurinn kastar teningnum = neutral
• Það er andstæðingurinn sem kastar teningnum = specifically emphasizing that it is the opponent, not someone else

So if you are correcting someone, contrasting players, or highlighting who has the turn, the cleft version is especially natural.