Hún athugaði gasflöskuna frekar en að kveikja upp.

Because gasflöskuna is the definite accusative singular form of gasflaska (gas cylinder / gas bottle).

Here is the breakdown:

• gasflaska = an indefinite singular noun
• gasflaskan = the definite nominative singular, the gas cylinder
• gasflöskuna = the definite accusative singular

It appears in the accusative because it is the direct object of athugaði (checked / examined).

A useful pattern with many feminine nouns is:

• nominative singular definite: -an
• accusative singular definite: -una

So:

• gasflaskan = the gas cylinder (subject form)
• gasflöskuna = the gas cylinder (object form)

Athugaði is the past tense of the verb að athuga, which means to check, to examine, or to look into.

So:

• að athuga = to check
• ég athuga = I check / am checking
• hún athugaði = she checked

In this sentence, hún athugaði simply means she checked.

Frekar en að means rather than or instead of when it is followed by an infinitive verb.

So in this sentence:

• frekar en að kveikja upp = rather than light a fire / rather than start lighting up

A very useful distinction is:

• frekar en
  • noun/pronoun = rather than
• frekar en að
  • infinitive = rather than doing / rather than to do

Examples:

• te frekar en kaffi = tea rather than coffee
• fara heim frekar en að bíða = go home rather than wait

Because kveikja is in the infinitive, and Icelandic often uses að as the infinitive marker, similar to English to.

Here:

• að kveikja upp = to light up / to start a fire

After the expression frekar en, Icelandic normally uses að before an infinitive verb:

• frekar en að gera eitthvað = rather than do something

So frekar en að kveikja upp is the natural way to say rather than lighting up / rather than starting the fire.

Að kveikja on its own usually means to light, to switch on, or to ignite something.

Adding upp creates the phrasal expression að kveikja upp, which often means:

• to light a fire
• to get a fire going
• to start the stove/fireplace

So the upp adds a more specific sense of starting up a fire or heating source, not just switching something on in a general sense.

Depending on context:

• kveikja ljósi = switch on a light
• kveikja upp = light the fire / get the stove going

Because upp is a verb particle that belongs with kveikja, and in Icelandic these particles are often placed after the verb.

So:

• að kveikja upp is the full verbal expression

In an infinitive phrase, this word order is very normal:

• að kveikja upp
• að standa upp
• að fara út

English learners sometimes expect the parts to stay together in a different way, but in Icelandic this is standard.

In other contexts, frekar can mean rather, preferably, or sometimes something like more readily. But in the expression frekar en, it works as part of a set phrase meaning:

• rather than
• instead of

So in this sentence, you should understand frekar en að as one unit, not as a separate more plus than idea.

Yes. Gasflaska is a compound noun:

• gas = gas
• flaska = bottle

So literally it is something like gas bottle, though in English gas cylinder may be a more natural translation depending on context.

Compound nouns are extremely common in Icelandic, and the last part usually determines the grammatical gender. Since flaska is feminine, gasflaska is also feminine.

Yes, Icelandic word order is somewhat flexible, but the given version is very natural.

The sentence:

• Hún athugaði gasflöskuna frekar en að kveikja upp.

clearly means:

• She checked the gas cylinder rather than lighting the fire.

If you moved things around, the sentence might still be grammatical, but the focus could change slightly or it could sound less natural in neutral speech.

For a learner, the safest thing is to keep this pattern:

• subject + past verb + object + frekar en að + infinitive phrase

It mainly expresses choice/preference in action, not just sequence.

So it means something like:

• She checked the gas cylinder instead of lighting the fire
• She chose to check the gas cylinder rather than light the fire

If you wanted to say simply that one thing happened before another, Icelandic would usually use a different structure.

Because hún is in the normal subject position at the start of the sentence, and the verb athugaði agrees with a third-person singular subject.

So:

• Hún = she
• athugaði = checked

Together:

• Hún athugaði = She checked

Also, gasflöskuna is marked as an object by its accusative form, which helps confirm that hún is the subject.