Myndin olli henni vonbrigðum, því hún var allt of stutt.

Because -in is the suffixed definite article in Icelandic.

• mynd = movie / picture / image depending on context
• myndin = the movie

So Icelandic often says the by attaching it to the noun, instead of using a separate word like English does.

Olli is the 3rd person singular past tense of the verb að valda.

This verb is a bit irregular:

• infinitive: valda
• present: veldur
• past: olli

So Myndin olli ... means The movie caused ...

In this sentence, that becomes the natural English idea The movie disappointed her.

Because the verb að valda takes the dative.

The pronoun forms are:

• nominative: hún = she
• accusative: hana = her
• dative: henni = her

Since valda requires dative, Icelandic uses henni, not hana.

Vonbrigðum is the dative plural form of vonbrigði.

A useful thing to know is that vonbrigði is normally used only in the plural, even when English would just say disappointment.

Forms you may meet:

• nominative/accusative plural: vonbrigði
• dative plural: vonbrigðum
• genitive plural: vonbrigða

Here it is vonbrigðum because it is part of the construction with valda.

Yes. It is a very common expression.

Literally, it is something like:

cause someone disappointment(s)

But in natural English, you usually translate it as:

disappoint someone

So:

• Myndin olli henni vonbrigðum
  = The movie disappointed her

Even though Icelandic uses a noun phrase, English usually prefers the verb disappoint.

Because this construction commonly uses two dative elements:

• the affected person: henni
• the thing caused: vonbrigðum

So the pattern is:

valda + person in dative + thing in dative

This is one of those patterns that learners usually just need to memorize as a set: valda einhverjum vonbrigðum

Henni vonbrigðum is the most natural order here.

A big reason is that short pronouns like henni often come earlier in the sentence than a fuller noun phrase. Icelandic frequently places light, unstressed pronouns before heavier complements.

So this order feels very normal:

• Myndin olli henni vonbrigðum

Other orders may be possible in special contexts, but this is the neutral one to learn first.

Here því means because.

It introduces the reason:

• því hún var allt of stutt
  = because it was far too short

So in this sentence, því is a conjunction.

Be aware that því can also appear in other roles in Icelandic, so its meaning depends on context.

Yes. In many contexts, af því að can also mean because.

So these are very close in meaning:

• ..., því hún var allt of stutt
• ..., af því að hún var allt of stutt

Very roughly:

• því can feel a bit shorter and more written/compact
• af því að can feel a bit more explicit and conversational

Both are common and correct.

Because mynd is a feminine noun.

In Icelandic, pronouns agree with the grammatical gender of the noun they refer to. So when hún refers back to myndin, it means it, not she in the natural English sense.

That is very normal in Icelandic:

• mynd is feminine
• therefore the pronoun is hún

Because the adjective agrees with the subject.

The adjective is stuttur = short, and its singular nominative forms are:

• masculine: stuttur
• feminine: stutt
• neuter: stutt

Since hún refers to myndin, which is feminine singular, the adjective must match that: stutt.

So even though stutt also looks like the neuter form, here it is the correct feminine singular form too.

Allt of means far too or much too.

Compare:

• of stutt = too short
• allt of stutt = far too short / much too short

So allt strengthens of.

Because the second part is a subordinate clause giving the reason:

• main clause: Myndin olli henni vonbrigðum
• reason clause: því hún var allt of stutt

In standard Icelandic writing, commas are commonly used to separate clauses like this. So the comma here is normal and expected in careful written Icelandic.