Hún geymir skjalið á minnislykli af því að fartölvan hennar er gömul.

Skjalið means the document, while skjal means just a document / document.

The ending -ið is the definite article attached to the noun. Icelandic usually adds the to the end of the noun instead of using a separate word like English does.

• skjal = document
• skjalið = the document

In this sentence, the speaker is referring to a specific document, so skjalið is used.

Because the preposition á can take different cases, and here it is followed by the dative.

In this sentence, á minnislykli means on a USB drive / on a memory stick in the sense of location or storage location. Since the document is being kept on the drive, not moved onto it, Icelandic uses the dative.

• nominative: minnislykill
• dative singular: minnislykli

A useful rule:

• á + accusative often shows motion toward / onto
• á + dative often shows location / position on

So:

• á minnislykil = onto a USB drive
• á minnislykli = on a USB drive

Minnislykill is the Icelandic word for a USB stick, memory stick, or flash drive.

It is a compound word:

• minni = memory
• lykill = key

So literally it is something like memory key.

This kind of compound noun is very common in Icelandic.

Geymir is from the verb að geyma, which usually means to keep, to store, or to save.

In this sentence, it means something like:

• she keeps the document
• she stores the document
• possibly she saves the document

The exact English translation depends on context. Icelandic geyma often emphasizes that something is being kept somewhere.

Also, geymir is the 3rd person singular present tense, so it matches hún:

• ég geymi = I keep/store
• þú geymir = you keep/store
• hún geymir = she keeps/stores

Af því að is a very common way to say because in Icelandic.

Literally, it is built from:

• af = of / from
• því = that / it, in a case form
• að = that / to

But as a full expression, you should learn af því að as a fixed phrase meaning because.

You may also see:

• því að = because

Both are common, but af því að is extremely natural in everyday Icelandic.

In Icelandic, possessive words like hennar (her) and hans (his) very often come after the noun.

So:

• fartölvan hennar = her laptop

This is the normal and natural word order.

Compare:

• bíllinn hans = his car
• húsið þeirra = their house

English puts the possessive before the noun, but Icelandic often puts these pronoun possessives after it.

Fartölvan hennar is the usual way to say her laptop.

The pattern is:

• noun + definite article + possessive pronoun

So:

• fartölva = laptop
• fartölvan = the laptop
• fartölvan hennar = her laptop

A form like hennar fartölva is not the normal everyday pattern for simple possession. Learners should generally use fartölvan hennar.

The adjective has to agree with the noun it describes.

The noun here is fartölvan:

• singular
• feminine
• nominative

So the adjective must also be:

• singular
• feminine
• nominative

That gives gömul.

Compare the forms:

• gamall = old, masculine singular nominative
• gömul = old, feminine singular nominative
• gamalt = old, neuter singular nominative

Examples:

• bíllinn er gamall = the car is old
• fartölvan er gömul = the laptop is old
• húsið er gamalt = the house is old

Because fartölvan hennar is the subject of the clause fartölvan hennar er gömul.

In Icelandic, the subject is usually in the nominative case. Here, the clause means her laptop is old, so fartölvan is the thing being talked about, and that makes it nominative.

Also:

• er = is
• predicate adjective = gömul

This is a standard subject + verb + adjective structure.

No. Á can take either the accusative or the dative, depending on meaning.

A very common rule is:

• dative for location: where something is
• accusative for motion/direction: where something goes

In this sentence:

• á minnislykli = on a memory stick, so it is location/storage
• therefore dative

Examples:

• Bókin er á borðinu. = The book is on the table.
  
  • location → dative
• Hún leggur bókina á borðið. = She puts the book onto the table.
  
  • motion toward → accusative

Yes, it is a subordinate clause introduced by af því að = because.

The clause is:

• fartölvan hennar er gömul

In Icelandic, this clause keeps a normal subject-verb order here:

• subject: fartölvan hennar
• verb: er
• complement: gömul

So for a learner, it is best to read it as:

• because her laptop is old

Icelandic word order can change in some contexts, but this sentence uses a very straightforward pattern.

Yes, that is a very natural interpretation.

The verb að geyma often means to keep/store, but in context it can overlap with English save, especially when talking about files and storage.

So possible natural translations include:

• She keeps the document on a USB stick because her laptop is old.
• She stores the document on a USB stick because her laptop is old.
• She saves the document on a USB stick because her laptop is old.

The exact best translation depends on the context, but the Icelandic grammar stays the same.