Við förum ekki út þegar það er myrkur.

Icelandic main clauses are V2 (the finite verb is in second position). The order here is:

• Subject: Við
• Finite verb: förum
• Negation/adverbs: ekki
• Verb particle/directional adverb: út So Við förum ekki út is the normal order. Putting ekki before the verb or at the very end would be ungrammatical or at least very odd in neutral word order.

• út = motion to the outside (go out). Use it with verbs of movement: fara út, hlaupa út, koma út.
• úti = location outside (be outside). Use it with stative verbs: vera úti, sitja úti.

In this sentence, förum is motion, so you need út.

það is a dummy or expletive subject (like English “it” in “it is cold”). Icelandic generally requires a subject, especially in weather/ambient statements: það er kalt, það rignir, það er myrkur.

You can sometimes omit það in a stylistic way (e.g., þegar dimmt er), but the neutral, everyday form includes það: þegar það er myrkur/dimmt.

• myrkur is a neuter noun meaning “darkness,” so það er myrkur literally means “there is darkness/it is dark.” This is a common set phrase.
• myrkt is the neuter singular form of the adjective meaning “dark.”
• dimmt is another very common adjective meaning “dim/dark.”

All of these are idiomatic:

• það er myrkur (noun; very common),
• það er myrkt (adjective; also common),
• það er dimmt (adjective; very common and perhaps the most everyday).

fara is irregular (strong):

• Present: fer (1sg/3sg), ferð (2sg), förum (1pl), farið (2pl), fara (3pl)
• Past: fór, fórst, fór, fórum, fóruð, fóru
• Supine (with hafa): farið (e.g., ég hef farið)

förum is 1st person plural present: “we go.”

No. þegar … þá … is common in speech and adds a bit of emphasis or clarity, but þá is optional. Your sentence is fine without it:

• Þegar það er myrkur (þá) förum við ekki út.

A handy order for main clauses is: Subject – finite verb – sentence adverbs – ekki – verb particles/directional adverbs – objects/PPs – time.

Examples:

• Við förum líklega ekki út í garðinn í kvöld.
• Þegar það er myrkur förum við alls ekki út. Avoid putting ekki after út at the end: Við förum út ekki sounds wrong.

út here is a directional adverb/particle. To specify direction with a destination or source, combine it with a preposition:

• út í
  • accusative: motion into an open area (e.g., út í garðinn = out into the yard)
• út á
  • accusative: motion onto a surface/area (e.g., út á götu = out onto the street)
• út úr
  • dative: motion out of an enclosed space (e.g., út úr húsinu = out of the house)

Be careful with út af: it often means “because of” or “off (down) from,” not simply “out of.”

• Við: final ð like English “th” in “this.”
• förum: ö like French “eu”; roll the r; the last syllable is short.
• ekki: kk is pre-aspirated; it sounds a bit like “eh-ki” with a little breath before the k.
• út: long ú like “oo” in “food,” followed by a clear t.
• þegar: þ like “th” in “thing”; roll the r; stress on the first syllable.
• það: final ð pronounced (and often links to the next word).
• myrkur: y sounds like i; the cluster rk is crisp, with a trilled/voiceless r before k.

Primary stress in Icelandic is on the first syllable of words.

Við is the nominative subject form (we). Okkur is the object/dative form (us). Subjects normally appear in the nominative in Icelandic, so you need við with förum:

• Subject: við
• Object/dative: okkur
• Genitive: okkar