Hundurinn reif töskuna.

Because Hundurinn is the noun hundur meaning dog with the definite article added to the end.

• hundur = dog
• hundurinn = the dog

In Icelandic, the is often attached to the noun as a suffix instead of being a separate word.

It is part of the definite form of hundur.

For this masculine noun:

• hundur = dog
• hundurinn = the dog

More specifically, Hundurinn here is nominative singular definite. That means it is:

• singular: one dog
• definite: the dog
• nominative: the subject of the sentence

Because töskuna is the accusative definite singular form of taska.

Here are the key forms:

• taska = a bag / bag (nominative singular)
• taskan = the bag (nominative singular definite)
• tösku = a bag (accusative singular)
• töskuna = the bag (accusative singular definite)

In this sentence, the bag is the direct object, so Icelandic uses the accusative form.

This is a normal vowel change in Icelandic called u-umlaut.

The a in taska changes to ö in forms like tösku and töskuna because of the ending pattern. You do not need to think about the historical details every time; the important thing is to learn these forms as part of the noun’s declension.

So for learners, it is best to remember:

• taska
• tösku
• töskuna

as related forms of the same word.

Because Icelandic marks grammatical role with case.

In this sentence:

• Hundurinn is the subject: the dog is doing the action.
• töskuna is the direct object: the bag is receiving the action.

So Icelandic uses:

• nominative for the subject: Hundurinn
• accusative for the direct object: töskuna

English mostly shows this with word order, but Icelandic often shows it directly on the noun.

Reif is the past tense of the verb rífa.

• rífa = to tear, to rip
• reif = tore, ripped

So the sentence is in the past: the action already happened.

No. Rífa is a strong verb, so its past tense is not made by simply adding a regular ending.

A useful set of forms is:

• rífa = infinitive, to tear
• reif = past singular
• rifu = past plural
• rifið = past participle

So reif has to be learned as part of the verb’s pattern.

Yes. Icelandic verbs agree with the subject in person and number.

Here, Hundurinn is third person singular, so the verb is also singular:

• Hundurinn reif töskuna. = The dog tore the bag.

If the subject were plural, the verb would change:

• Hundarnir rifu töskuna. = The dogs tore the bag.

Yes. Icelandic word order is more flexible than English, although Hundurinn reif töskuna is the most neutral order here.

You could also say:

• Töskuna reif hundurinn.

That still means roughly The dog tore the bag, but it puts more emphasis on the bag.

In main clauses, Icelandic often follows a verb-second pattern, so when something else comes first, the finite verb usually stays in the second position.

A rough learner-friendly guide is:

• Hundurinn ≈ HUN-du-rin
• reif ≈ rayf
• töskuna ≈ TUR-sku-na or TOSK-u-na with a rounded vowel

A few helpful points:

• Stress is usually on the first syllable of each word.
• ei in reif sounds roughly like ay in day, though not exactly the same.
• ö in töskuna is not a normal English vowel, so it may take practice.

The indefinite version would be:

• Hundur reif tösku.

That means A dog tore a bag.

Compare:

• hundur = a dog
• hundurinn = the dog
• tösku = a bag (accusative)
• töskuna = the bag (accusative definite)

This is a very useful comparison because it shows both definiteness and case at the same time.

Not necessarily. Taska is a general word for a bag, and in context it could mean a handbag, school bag, or another kind of bag.

The exact English translation depends on context. In a simple learner sentence like this, bag is usually the safest translation.