Tókstu eftir því að ljósin voru slökkt þegar við komum heim?

Tókstu means did you take / did you notice here, because the verb taka eftir means to notice.

It comes from the question form of þú tókst:

• statement: Þú tókst eftir því ...
• question: Tókstu eftir því ...?

In yes/no questions, Icelandic normally puts the finite verb first. With þú, the pronoun is very often shortened and attached to the verb, so tókst þú becomes tókstu in normal usage.

So Tókstu ...? is basically Did you ...?

Because taka eftir is a fixed Icelandic expression. You should learn it as a whole:

• taka eftir = notice, observe, pay attention to

This is one of those cases where a word-for-word translation does not help much. Even though taka usually means take, the full phrase taka eftir has its own meaning.

Examples:

• Ég tók ekki eftir þessu. = I didn’t notice this.
• Tókstu eftir henni? = Did you notice her?

In tókstu eftir því að ..., the word því is part of a very common pattern:

• taka eftir því að + clause = notice that + clause

So the sentence structure is:

• Tókstu eftir því = Did you notice it / that
• að ljósin voru slökkt = that the lights were off

The því is there because eftir in this expression takes the dative, and því is the dative form of það.

So even though English just says notice that..., Icelandic usually says something more like notice of that that... structurally.

Here að means that and introduces a subordinate clause:

• að ljósin voru slökkt = that the lights were off

This is not the infinitive-marker að as in að borða (to eat). It is the conjunction that.

So:

• Tókstu eftir því að ljósin voru slökkt?
• Did you notice that the lights were off?

Ljósin means the lights. The ending -in is the definite article attached to the noun.

So:

• ljós = lights / light in an indefinite sense
• ljósin = the lights

In this sentence, we are talking about specific lights, so the definite form makes sense.

Also, ljós is a neuter noun, and ljósin here is neuter plural definite.

Because slökkt agrees correctly with ljósin.

The important point is that ljósin is neuter plural, and the neuter plural form of this adjective/participle is also slökkt. So it may look singular to an English speaker, but it is actually the correct agreeing form.

So:

• ljósið var slökkt = the light was off
• ljósin voru slökkt = the lights were off

The form slökkt happens to be the same in neuter singular and neuter plural.

Voru slökkt means were off or were switched off.

This describes a state, not the action of going out.

That matters because Icelandic can express a different idea if the lights themselves went out:

• Ljósin voru slökkt. = The lights were off.
• Ljósin slokknuðu. = The lights went out.

So in your sentence, the speaker is describing the condition of the lights when they got home, not the moment when they turned off.

Because þegar introduces a subordinate clause, and subordinate clauses do not use the normal main-clause verb-second pattern.

So after þegar, Icelandic keeps the subject before the verb:

• þegar við komum heim = when we came home

Compare:

• main clause question: Komum við heim? = Did we come home?
• subordinate clause: þegar við komum heim = when we came home

This is very normal Icelandic subordinate-clause word order.

That is a very good question, because komum can look the same in the present and past for við.

Here, context tells you it is past:

• ljósin voru slökkt is already in the past
• the whole situation is about what happened when we got home

So þegar við komum heim is understood as when we came home, not when we come home.

This kind of ambiguity is normal in Icelandic, and context usually makes the meaning clear.

Because heim by itself already means homeward / home as a direction.

So Icelandic says:

• koma heim = come home
• fara heim = go home

You do not need a separate word like to.

A useful contrast is:

• heim = home, to home (direction)
• heima = at home (location)

So:

• Við komum heim. = We came home.
• Við vorum heima. = We were at home.