Hún batt hundinn.

Because hundinn is the accusative singular definite form of hundur (dog).

In Hún batt hundinn, the dog is the direct object of the verb, so Icelandic uses the accusative case.

Here are the forms:

• hundur = a dog / dog (nominative)
• hundurinn = the dog (nominative)
• hund = a dog (accusative)
• hundinn = the dog (accusative)

So:

• Hundurinn beit hana. = The dog bit her.
  The dog is the subject, so nominative is used.
• Hún batt hundinn. = She tied the dog.
  The dog is the object, so accusative is used.

Batt is the past tense, 3rd person singular form of the verb binda, which means to tie or to bind.

So:

• að binda = to tie
• ég batt = I tied
• þú battst = you tied
• hún batt = she tied

This is a strong verb, so its past tense is not made in a simple regular way.

Because the subject is hún (she), and in the past tense Icelandic uses the form batt for 3rd person singular.

In this sentence:

• Hún = she
• batt = tied

So Hún batt means She tied.

The same verb form is also used with hann (he) and það (it) in the singular past:

• Hann batt hundinn.
• Hún batt hundinn.
• Það batt hundinn.
  (grammatically possible, even if the meaning may be odd)

Yes. The word is hún, with ú.

The accent matters in Icelandic spelling. Vowels with accents are treated as distinct letters, not just optional marks.

So you should write:

• hún = she

not:

• hun

In Icelandic, the word for the is usually added to the end of the noun as a suffix.

So instead of a separate word like English the dog, Icelandic often says:

• hundurinn = the dog
• hundinn = the dog (in the accusative)

This is a very common feature of Icelandic grammar.

The basic word order here is:

Subject – Verb – Object

• Hún = subject
• batt = verb
• hundinn = object

So the structure matches normal English order:

• She tied the dog.

This makes the sentence quite straightforward for an English speaker.

You know hún is the subject because:

1. It is the pronoun doing the action.
2. It is in the nominative case.
3. The verb agrees with a 3rd person singular subject.

Meanwhile, hundinn is marked as the object by its accusative form.

So the grammar tells you who did the action and who received it.

The spelling has tt, but Icelandic pronunciation does not work exactly like English spelling.

For a learner, it is enough at first to know that:

• batt is pronounced with a clear final t sound
• the vowel is short
• the double consonant affects pronunciation, but not in quite the same way as simply “holding” the t longer in English

A rough learner-friendly pronunciation is something like baht with a short vowel.

The dictionary form is hundur.

When you look up nouns in Icelandic, you usually look for the nominative singular indefinite form:

• hundur = dog

Forms like hundinn are inflected forms that show case and definiteness.

It is definite, because hundinn means the dog, not a dog.

Compare:

• Hún batt hund. = She tied a dog.
• Hún batt hundinn. = She tied the dog.

The ending -inn shows definiteness here.

Usually in a sentence like this, yes, the most natural meaning is to tie in a physical sense.

So Hún batt hundinn would normally be understood as something like:

• she tied up the dog
• she fastened the dog

Depending on context, binda can also be used in broader senses, but the physical meaning is the most likely here.

Yes. That is a very natural English translation.

The Icelandic sentence itself does not force the extra word up, but English often uses tie up where Icelandic simply uses binda.

So both of these can work in English, depending on context:

• She tied the dog.
• She tied up the dog.