Forritið uppfærist ekki án rafmagns.

-ið is the suffixed definite article for a neuter singular noun in Icelandic.

• forrit = a program
• forritið = the program
  Here it’s also in the nominative case because it’s the subject of the sentence.

uppfærist is the present tense, 3rd person singular form of the verb uppfærast (the -st form). The base forms you’ll often see are:

• uppfæra = to update (something) (active/transitive)
• uppfærast = to get updated / update itself (middle/passive-like -st form)

So Forritið uppfærist literally behaves like The program updates itself / gets updated.

Not exactly the same as English passive, but it’s often used with a similar meaning. Icelandic -st verbs commonly express:

• something happening “by itself,” or
• an impersonal/passive-like idea where the agent isn’t mentioned.

Compare:

• Forritið uppfærist. = The program updates (itself) / gets updated.
• Forritið er uppfært. = The program is updated (more like a result/state; uses vera
  • past participle).
    Both can be translated similarly, but they’re not always interchangeable in nuance.

Yes, that’s a possible alternative:

• Forritið uppfærir sig ekki ... uses the active verb uppfæra plus the reflexive pronoun sig (itself).
• Forritið uppfærist ekki ... packages that idea into the -st form and is often more natural/compact.

Both mean basically the same thing here.

In a simple main clause, Icelandic typically places the finite verb early, and ekki usually comes after that verb:

• Forritið uppfærist ekki ...
  This is normal Icelandic word order for negation (roughly: subject + verb + ekki
  • the rest).

The preposition án (without) governs the genitive case.
So rafmagn (electricity/power) changes to its genitive singular form rafmagns:

• nominative/accusative: rafmagn
• genitive: rafmagns

That’s why it’s án rafmagns (not án rafmagn).

It’s indefinite. In Icelandic, after án, you often see an indefinite genitive noun when speaking generally:

• án rafmagns = without electricity / without power (in general)

You could make it definite in some contexts (án rafmagnsins), but that would sound like you mean a specific, known electricity supply (more specific than the general idea).

Stress in Icelandic is usually on the first syllable of a word:

• FOR-rit-ið
• UPP-fær-ist
• RAF-magns

A few practical pronunciation notes:

• upp- often sounds like a crisp up(p) with a clear p.
• æ in uppfærist is like the vowel in English eye (approx.).
• The final -s in rafmagns is pronounced (a clear s sound).

You can, but it changes the feel.

• Forritið = the program (a specific one)
• Forrit (without article) = a program / programs (in general) depending on context

So dropping the article can make it sound more like a general statement about “a program” rather than “the (particular) program.”