Andstæðingurinn tapaði.

Andstæðingurinn is the noun andstæðingur with the definite article attached.

• andstæðingur = opponent
• -inn = the

So andstæðingurinn means the opponent.

Because Icelandic usually adds the to the end of the noun, you get one word instead of two separate words.

That is a normal feature of Icelandic grammar. The definite article is usually a suffix, not a separate word.

So:

• andstæðingur = an opponent / opponent
• andstæðingurinn = the opponent

English uses a separate article, but Icelandic very often puts it on the end of the noun.

In this sentence, the noun is the subject of the verb, so it is in the nominative case.

• andstæðingur = nominative singular indefinite
• andstæðingurinn = nominative singular definite

If you changed the case, the ending would change too. For example, andstæðinginn is an accusative form, so it would not fit here as the subject of this sentence.

Tapaði is the past tense of the verb tapa.

So:

• tapa = to lose
• tapaði = lost

Here it means lost in the simple past.

In Icelandic, verbs usually do show person distinctions in some tenses, but in the past singular many forms are the same.

So tapaði can be used with different singular subjects depending on context. In this sentence, the subject is andstæðingurinn, so the meaning is simply the opponent lost.

Here tapa is being used intransitively, meaning simply to lose in the sense of to be defeated or not win.

So the sentence just says that the opponent lost.

In other contexts, tapa can also take an object and mean lose something, as in lose money, lose a game, and so on. But in this sentence, no object is needed.

No, this is a very basic, neutral sentence pattern:

• Subject: Andstæðingurinn
• Verb: tapaði

So it is just Subject + Verb, which is very natural here.

Icelandic does have verb-second word order in many situations, but this sentence is a straightforward example and does not feel unusual at all.

A few useful pronunciation points:

• æ sounds roughly like eye
• ð sounds like the th in this
• stress usually falls on the first syllable in Icelandic

So andstæðingurinn can be approached roughly as:

AHND-steye-dhing-ur-rinn

That is only an approximation, but it helps with the main sounds.

Again, a rough guide:

• tapaði is approximately TAH-pa-thi
• the ð is the soft th sound from this
• the stress is on the first syllable: TA

So the rhythm is roughly TA-pa-thi.

Yes, that can be a natural English translation in some contexts, but the Icelandic sentence itself is more directly The opponent lost.

Was defeated is a slightly more interpretive translation. It often works well in English, especially in sports or competition contexts, but the basic sense of the Icelandic is simply that the opponent lost.

Yes, but it would mean something different.

• Andstæðingurinn tapaði = The opponent lost
• Andstæðingur tapaði = An opponent lost / Opponent lost depending on context

So the version with -inn is definite, and the version without it is indefinite.