Learn Hardcore Icelandic: Lampinn er slökktur í kvöld.

The ending -inn is the masculine nominative singular definite article.
So Lampinn er slökktur í kvöld literally starts with “The‑lamp is off tonight.”

What does er slökktur literally mean? Is it a passive form?

er = is (3rd person singular of vera “to be”)
slökktur = past participle of slökkva (to turn off, extinguish), used adjectivally

So er slökktur is literally “is turned‑off” and describes a state, not the action of turning it off.

Grammatically, it looks like a passive (“is turned off”), but in Icelandic this kind of structure with vera + past participle is usually understood as a resulting state, very much like English “The lamp is broken / is closed / is locked.”

Why is the verb in the present tense (er) when English might say “The lamp will be off tonight”?

Icelandic very often uses the present tense + a time expression to talk about the future.

Lampinn er slökktur í kvöld.
= The lamp is off tonight / The lamp will be off tonight.

The future meaning comes from í kvöld (“this evening / tonight”), not from a special future tense. This is normal and very common in Icelandic.

What exactly does í kvöld mean? Is it “in the evening” or “tonight”?

Í kvöld usually means “this evening” or “tonight” (later today, after late afternoon).

It does not mean “in the evenings” in general (habitually). For that you would normally say:

á kvöldin = in the evenings (on evenings in general, as a habit)

So:

Lampinn er slökktur í kvöld.
= The lamp is (going to be) off this evening / tonight (just this one time).
Lampinn er slökktur á kvöldin.
= The lamp is off in the evenings (as a regular practice).

Why is it í kvöld and not á kvöld or í kvöldina?

For “this evening / tonight”, Icelandic uses the set phrase:

í kvöld (literally “in evening”)

You do not normally add a definite ending here (so not í kvöldina), because time adverbials like í dag, í gær, í kvöld behave more like fixed adverbs than like regular “in the X” phrases.

Á kvöldin is a different expression:

á kvöldin = in the evenings (habitually, every evening / most evenings)

So í kvöld is one specific evening (tonight), and á kvöldin is evenings in general.

Why is the adjective slökktur and not some other form like slökkt?

Slökktur is the masculine nominative singular form of the past participle of slökkva.

Past participle forms (used adjectivally) are:

Masculine: slökktur
Feminine: slökkt
Neuter: slökkt

The adjective has to agree with the noun in gender, number, and case:

Lampinn er slökktur.
- lampinn = masculine nominative singular → slökktur
Peran er slökkt. (The bulb is off.)
- peran = feminine nominative singular → slökkt
Ljósin eru slökkt. (The lights are off.)
- ljósin = neuter plural nominative → participle in neuter plural, also slökkt in practice here

So Lampinn er slökktur is chosen because lampinn is masculine.

Could I say Lampinn er slökkt instead of Lampinn er slökktur?

For standard grammar, no.

Lampinn is masculine, nominative singular.
The participle/adjective must match this: slökktur (masc. nom. sg.).

You use slökkt with feminine or neuter nouns:

Peran er slökkt. (fem.)
Ljós er slökkt. (neut.)

So Lampinn er slökktur is the correct agreement.

Why is Lampinn in the nominative case here?

In “Lampinn er slökktur”, lampinn is the subject of the verb vera (er):

[Lampinn] (subject) er [slökktur] (predicate adjective).

Subjects of finite verbs are normally in the nominative case in Icelandic.

If you changed the structure, the case might change. For example:

Það er slökkt á lampanum.
- There is (it is) turned off on the lamp / The lamp is turned off.
- Here lampanum is in the dative, because of the preposition á with this verb phrase.

But in Lampinn er slökktur í kvöld, lampinn is just a simple nominative subject.

Is slökktur here really an adjective, or is it still a verb form?

Historically it’s a past participle of the verb slökkva.
Functionally, in this sentence it behaves like an adjective:

It agrees with lampinn in gender/number/case.
It describes the state the lamp is in.

So you can think of it like English “broken”, “closed”, “locked”: originally verbal, but here used adjectivally:

Lampinn er slökktur.
= The lamp is off / is in an “extinguished/turned‑off” state.

How would I say “The lamp is on tonight” as the opposite of this sentence?

You can mirror the structure with the participle of kveikja (to turn on, light):

Lampinn er kveiktur í kvöld.
= The lamp is on tonight.

Again, kveiktur is the masculine nominative singular form, agreeing with lampinn.

You will also often hear another very common pattern:

Það er kveikt á lampanum.
Literally: There is lit on the lamp. ≈ The lamp is on.

Both patterns are natural; your original sentence is using the “Lampinn er + participle” pattern.

Can the word order change? For example, can I say Í kvöld er lampinn slökktur?

Yes, Icelandic allows some flexibility in word order for emphasis, as long as the verb stays in second position in a main clause.

Both of these are grammatical:

Lampinn er slökktur í kvöld.
(Neutral, common order: subject–verb–predicate–time.)
Í kvöld er lampinn slökktur.
(Emphasis on “Tonight”; literally: Tonight, the lamp is off.)

In both cases, er is in the second position, which is required in Icelandic main clauses.

Could Lampinn er slökktur í kvöld mean something habitual, like “The lamp is off in the evenings”?

No, not by itself. As written, it refers to one specific evening (tonight).

For a habitual meaning, you would normally change the time expression:

Lampinn er slökktur á kvöldin.
= The lamp is off in the evenings (as a rule / habit).

So:

í kvöld → this evening / tonight (one time)
á kvöldin → in the evenings (generally, habitually)