Hún skellir hurðinni.

Skellir is the 3rd person singular present tense form of the verb að skella.

So:

• að skella = the dictionary form, to slam / bang shut
• hún skellir = she slams

A useful pattern to notice is that many Icelandic verbs have these present-tense singular endings:

• ég skelli
• þú skellir
• hún/hann/það skellir

So the -ir ending here matches hún because the subject is 3rd person singular.

Because in this meaning, að skella commonly takes a dative object in Icelandic.

That is something learners often just have to memorize with the verb:

• skella e-u = slam something
  where e-u stands for a noun in the dative

So:

• hurð = door
• hurðinni = the door in the dative singular

An English speaker may expect an accusative-style direct object, but Icelandic verbs often govern a specific case, and it is not always the one English would suggest.

Here -inni shows that the noun is:

• singular
• definite = the
• dative
• feminine

So hurðinni means roughly the door, but specifically in the form required by the verb.

For this noun:

• hurð = a door
• hurðin = the door, nominative
• hurðina = the door, accusative
• hurðinni = the door, dative

So the ending is doing a lot of grammatical work at once.

Because Icelandic usually puts the definite article onto the end of the noun instead of using a separate word like English the.

So:

• hurð = door
• hurðin / hurðina / hurðinni = the door, with different case forms

Icelandic does have separate article forms such as hinn / hin / hið, but in an ordinary sentence like this, the normal pattern is the suffixed article attached to the noun.

Hún is the nominative singular feminine pronoun, so it is the subject form meaning she.

It is used because the sentence has a feminine subject.

Also, in Icelandic, pronouns can refer not only to people but also to nouns with grammatical gender. So in context, hún could sometimes refer back to a feminine noun, where English might say it. But by itself, most learners will naturally read hún as she.

Yes. Skella is stronger and more vivid than a neutral verb like loka.

Compare:

• loka hurðinni = close the door
• skella hurðinni = slam the door

So skella usually suggests:

• force
• abrupt movement
• often noise
• sometimes emotion, such as anger or impatience

Yes. Hún skellir hurðinni is a normal, straightforward subject–verb–object sentence:

• Hún = subject
• skellir = verb
• hurðinni = object

But Icelandic also follows the verb-second pattern in main clauses. That means if you move another element to the front, the finite verb still stays in second position:

• Nú skellir hún hurðinni. = Now she slams the door.

So the sentence you have is the most neutral order, but not the only possible one.

Not necessarily. The Icelandic present tense can be used for:

• something happening now
• something habitual
• a vivid description in a story

So Hún skellir hurðinni could mean:

• she is slamming the door
• she slams the door
• she habitually slams the door

The exact meaning depends on context.

The dictionary form is hurð.

That is the basic noun form you would look up. It is a feminine noun meaning door.

In this sentence, it appears as hurðinni because it has been changed to match:

• the required case: dative
• the number: singular
• the definiteness: the

So when looking vocabulary up, always strip a form like hurðinni back to its base form: hurð.

The accent in ú is not just a spelling decoration. It shows a different vowel sound from plain u.

So hún is pronounced with ú, and the accent matters for both spelling and pronunciation.

A useful general point:

• Icelandic accents usually mark vowel quality/length
• they do not mainly mark stress the way learners sometimes expect

So hún must be learned as its own form, not as just hun with extra emphasis.