Hundurinn snýr sér við þegar ég opna hurðina.

Icelandic usually puts the definite article on the end of the noun instead of using a separate word.

• hundur = dog

• hundurinn = the dog

• hurð = door
• hurðina = the door

So both Hundurinn and hurðina already include the meaning of the.

It is the common expression for turn around.

Breakdown:

• snýr = turns
• sér = himself / herself / itself
• við = a particle that helps give the sense of turning around

So grammatically it is something like turns himself around, but in natural English we just say turns around.

The base form is snúa sér við.

Because Icelandic uses a reflexive pronoun in this expression: snúa sér við.

A reflexive pronoun refers back to the subject:

• Ég sný mér við = I turn around
• Þú snýrð þér við = you turn around
• Hann/Hún/Hundurinn snýr sér við = he/she/the dog turns around

So sér refers back to Hundurinn.

Because this verb phrase takes the dative reflexive pronoun, not the accusative one.

The reflexive forms are:

• accusative: sig
• dative: sér
• genitive: sín

With snúa sér við, Icelandic uses the dative form, so sér is correct.

This is something you often just have to learn together with the verb or expression.

Here við is part of the expression snúa sér við. It is not the everyday preposition meaning with.

In this sentence, við works more like a particle, similar to how English uses around in turn around.

So you should learn snúa sér við as a unit.

Because hurðina is the accusative form, and hurð is the direct object of opna.

• hurðin = the door as a subject form (nominative)
• hurðina = the door as an object form (accusative)

Compare:

• Hurðin opnast. = The door opens.
  Here the door is the subject.

• Ég opna hurðina. = I open the door.
  Here the door is the object.

Since ég is doing the opening, hurðina has to be in the object form.

Because the sentence is describing something that happens generally or habitually: whenever I open the door, the dog turns around.

So the present tense works very naturally here.

If you wanted a past version, you could say:

• Hundurinn sneri sér við þegar ég opnaði hurðina.

That means the same kind of relationship, but in the past.

Yes. Icelandic has a verb-second pattern in main clauses.

So:

• Hundurinn snýr sér við þegar ég opna hurðina.

But if the when clause comes first:

• Þegar ég opna hurðina, snýr hundurinn sér við.

Notice that snýr comes before hundurinn in the main clause.

That word order is very important in Icelandic.

Sometimes, yes. Both can introduce a time clause, but þegar is the safest and most common choice for learners in a sentence like this.

So:

• þegar ég opna hurðina = when I open the door

You may see er in other contexts, especially in more formal or literary language, but þegar is the one to learn first for time expressions like this.

For a learner, it is best to treat it as a fairly fixed expression.

The normal order here is:

• snýr sér við

That is the most natural order in a simple statement like this.

You may see particle placement vary in more complex sentences, but if you keep snúa sér við together in this order, you will usually be safe.

Because it agrees with a third-person singular subject: Hundurinn.

The infinitive is snúa. Its present forms include:

• ég sný
• þú snýrð
• hann/hún/það snýr

Since hundurinn is grammatically like he/it, the sentence uses snýr.

So:

• Hundurinn snýr sér við = the dog turns around