Þau stöðvuðu bardagann.

Here, þau is plural.

You can tell from the verb stöðvuðu, which is the 3rd person plural past form of stöðva.

A useful extra note: in modern Icelandic, þau can sometimes be used as a singular gender-neutral pronoun, but that is not what is happening in this sentence, because the verb is clearly plural.

Þau is in the nominative case because it is the subject of the sentence — the ones doing the action.

In Þau stöðvuðu bardagann, þau are the ones who stopped something, so nominative is the expected case.

These are all forms of they, but they reflect grammatical gender:

• þeir = masculine plural
• þær = feminine plural
• þau = neuter plural

So þau tells you that the subject is being referred to as neuter plural.

That can happen for:

• neuter nouns
• groups of things
• sometimes groups of children
• some other contexts depending on how the referent is framed

If the sentence were about a masculine or feminine plural group, you would expect þeir or þær instead.

Stöðvuðu is the past tense, 3rd person plural form of the verb stöðva, meaning to stop.

A few related forms are:

• ég stöðvaði = I stopped
• þú stöðvaðir = you stopped
• hann/hún/það stöðvaði = he/she/it stopped
• við stöðvuðum = we stopped
• þið stöðvuðuð = you all stopped
• þau stöðvuðu = they stopped

So the sentence is straightforwardly in the past: they stopped ...

Because bardagann is the form that fits this sentence.

The base dictionary form is bardagi = battle.

Here, though, the word is:

• definite: the battle
• accusative singular: because it is the direct object

So:

• bardagi = battle / the form you look up in a dictionary
• bardagann = the battle (accusative singular definite)

The ending -ann contains the definite article — basically the meaning of English the — attached to the noun.

In Icelandic, the is very often added as a suffix instead of being a separate word.

So:

• bardaga = a battle / battle
• bardagann = the battle

That final -nn is a very common sign that you are looking at a definite noun form.

Because it is the direct object of the verb stöðva.

The sentence structure is:

• þau = subject
• stöðvuðu = verb
• bardagann = object

Since stöðva takes a direct object, that object appears in the accusative case.

So Icelandic is showing something English usually does not show clearly with noun endings:

• who is doing the action
• what is receiving the action

No — this is the normal, neutral word order:

Subject + Verb + Object

• Þau = subject
• stöðvuðu = verb
• bardagann = object

So this is a very standard sentence pattern.

Icelandic can change word order for emphasis or style, but this version is the most basic and natural one.

Yes, but it would usually change the focus or style.

For example, Bardagann stöðvuðu þau is possible, and it puts more emphasis on the battle.

However, the original Þau stöðvuðu bardagann is the most neutral way to say it.

So for a learner, the original sentence is the safest pattern to imitate.

The most important points are:

• Þ / þ is like th in thin
• ð is roughly like th in this
• au is roughly like oy in boy, though not exactly the same
• ö has no perfect English equivalent; it is a rounded front vowel, somewhat like German ö

So:

• Þau starts with a thin-type th
• stöðvuðu contains ð, the softer th sound

A rough learner-friendly approximation might be:

• Þau ≈ thoy
• stöðvuðu ≈ something like stur-thvuthu

Those are only approximations, but they can help you get started.