Hurðin opnast sjálfkrafa þegar við nálgumst endurvinnslustöðina.

Hurðin is hurð (door) with the definite article attached: -in = the (common for feminine nouns in the nominative singular).

• hurð = a door
• hurðin = the door
  In everyday Icelandic you often use the definite form when talking about a specific door in context (like the door of the building you’re approaching).

• opnar = opens (something) / opens (it) (active; usually needs an object or an implied one)
• opnast = opens (by itself) / gets opened (middle voice, often functioning like an intransitive or passive-ish meaning)

So Hurðin opnast is like The door opens (automatically / on its own), not The door opens something.

It’s the Icelandic middle voice (often marked with -st). It can express:

• something happening by itself (inchoative)
• something being done to the subject without naming an agent (passive-like)

Here it’s the natural “automatic door” sense: the door opens (no agent mentioned).

In a main clause Icelandic is usually V2 (the finite verb comes early), and adverbs like sjálfkrafa often come right after the verb if there’s no object:

• Hurðin opnast sjálfkrafa.

You can move it for emphasis, but opnast sjálfkrafa is the neutral, most common placement.

Yes, it’s historically a compound:

• sjálf- = self-
• krafa = demand/requirement (historically; the whole word has fossilized)

As a whole, sjálfkrafa means automatically, by itself, without manual action.

Because þegar introduces a subordinate clause, and in subordinate clauses Icelandic typically keeps subject before verb (no V2 inversion):

• þegar við nálgumst endurvinnslustöðina = when we approach the recycling station

Inversion like nálgumst við is typical of main-clause V2, not after a subordinating conjunction like þegar.

nálgumst = we approach (present tense).
It’s 1st person plural of the verb nálgast (to approach), which is commonly used in the -st form. Many -st verbs are middle-voice or “deponent-like” in everyday usage.

Roughly:

• infinitive: nálgast
• við nálgumst = we approach

Because it’s: 1) definite (the recycling station)
2) accusative singular (the verb nálgast commonly takes an accusative object of what you approach)
3) feminine noun (stöð is feminine)

So:

• endurvinnslustöð = a recycling station
• endurvinnslustöðina = the recycling station (accusative, definite)

It’s a typical Icelandic compound:

• endur- = re- / again
• vinnsla = processing
• stöð = station

So literally: re-processing station → natural English: recycling station. The -ina ending adds the + accusative feminine singular.

The present tense in Icelandic is used for:

• general/habitual truths (like how an automatic door works), and often also
• near future with context

Here it’s a general statement: The door opens automatically when we approach…
If you were telling a past story you’d likely use past tense:

• Hurðin opnaðist sjálfkrafa þegar við nálguðumst endurvinnslustöðina.

Common ones:

• ð in Hurðin: often like a soft th in this, but it can be very light or even disappear depending on position/speed.
• á in nálgumst: a diphthong, roughly like ow in now (but Icelandic is more crisp).
• ö in stöðina: a rounded vowel (not the same as English o), something like German ö.
• -st clusters (opnast, nálgumst): pronounced clearly; don’t drop the t.