Slökkvitækið er undir vaskinum og reykskynjarinn er í loftinu fyrir ofan hurðina.

In Icelandic, the is usually attached to the end of the noun instead of appearing as a separate word.

So:

• slökkvitæki = fire extinguisher
• slökkvitækið = the fire extinguisher

And:

• reykskynjari = smoke detector
• reykskynjarinn = the smoke detector

The same thing happens in other forms too, so you also get forms like vaskinum, loftinu, and hurðina, where the noun includes both case ending and definiteness.

Slökkvitækið is the neuter singular nominative definite form of slökkvitæki.

Breakdown:

• slökkvitæki = fire extinguisher
• -ið = the definite ending for a neuter noun in this form

It is the subject of the clause, so nominative is used:

• Slökkvitækið er... = The fire extinguisher is...

This is very common in Icelandic: the noun changes form depending on gender, number, case, and whether it is definite.

Because undir can take different cases depending on meaning.

• accusative: movement to a position under something
• dative: location already under something

Here the fire extinguisher is already located there, so Icelandic uses the dative:

• undir vaskinum = under the sink

The base noun is vaskur:

• nominative indefinite: vaskur
• accusative definite: vaskinn
• dative definite: vaskinum

So vaskinum is exactly what you expect after undir when the meaning is static location.

For the same reason as with undir: the preposition í changes case depending on whether there is movement or location.

• í
  • accusative = into
• í
  • dative = in / inside / located in

Here the smoke detector is already in its position, so Icelandic uses the dative:

• í loftinu

The noun is loft:

• loft = ceiling / air, depending on context
• loftinu = the ceiling, in the dative definite form

Yes, loft can mean both air and ceiling, and context tells you which one is meant.

In this sentence, í loftinu fyrir ofan hurðina clearly means in/on the ceiling above the door, so loftinu means the ceiling.

This is a very useful word to learn because it is common in both meanings:

• loftið = the air
• loftið = the ceiling

You have to rely on context.

This is partly just idiomatic usage. In English, we usually say something is on the ceiling, but Icelandic often says í loftinu for something mounted in the ceiling area.

So even if it feels more natural to an English speaker to think on, Icelandic commonly uses í here.

It is best to learn this as a natural collocation:

• reykskynjari er í loftinu = a smoke detector is on/in the ceiling

Fyrir ofan is a fixed expression meaning above or over.

So:

• fyrir ofan hurðina = above the door

It works like a two-word preposition. Even though it is made of two words, you should learn it as one unit of meaning:

• fyrir ofan = above
• fyrir neðan = below

These expressions are very common in Icelandic when describing location.

Because fyrir ofan takes the accusative.

The noun is hurð:

• nominative indefinite: hurð
• accusative definite: hurðina
• dative definite: hurðinni

So after fyrir ofan, Icelandic uses:

• fyrir ofan hurðina = above the door

This is a good example of something learners need to memorize with the preposition itself: not all location expressions use the same case.

Both are compounds, and Icelandic makes compound nouns very freely.

Slökkvitæki can be understood as:

• slökkvi- = extinguishing / putting out
• tæki = device, tool

So slökkvitæki is literally something like extinguishing device.

Reykskynjari can be broken down as:

• reykur / reyk- = smoke
• skynjari = detector / sensor

So reykskynjari is literally smoke detector.

Learning to spot compounds is very helpful in Icelandic, because many long words become much less intimidating once you see the parts.

Yes. This is very normal, neutral word order.

Structure:

• Slökkvitækið = subject
• er = verb
• undir vaskinum = location
• og
• reykskynjarinn = new subject
• er
• í loftinu fyrir ofan hurðina = location

So it is basically:

• The fire extinguisher is under the sink and the smoke detector is in the ceiling above the door.

Icelandic word order can be more flexible than English, but this sentence is straightforward and natural.

A few sounds may stand out to English speakers:

• ð in hurðina is like the th in this
• æ in slökkvitækið sounds roughly like eye
• ö in slökkvi- does not exist exactly in English; it is somewhat like the vowel in some pronunciations of burn, but more rounded
• j in skynjari is pronounced like English y

Very rough approximations:

• slökkvitækið ≈ SLUHK-kvi-ty-eye-thith
• reykskynjarinn ≈ rayk-skin-ya-rinn
• hurðina ≈ HUR-thi-na

Those are only approximations, but they can help you get started.