Ég athuga alltaf hvort slökkvitækið sé á sínum stað áður en gestirnir koma.

Here hvort means whether.

So Ég athuga alltaf hvort ... = I always check whether ...

A useful contrast is:

• hvort = whether / if in an indirect yes-no question
  
  • Ég veit ekki hvort hann kemur. = I don’t know whether he is coming.
• ef = usually if in a conditional sense
  
  • Ef hann kemur, förum við. = If he comes, we will go.

So in this sentence, hvort is the natural word because the speaker is checking a fact, not setting up a condition.

sé is the subjunctive form of vera (to be).

• er = indicative, the normal form for stating a fact
• sé = subjunctive, often used when something is uncertain, being checked, reported, wished for, etc.

In this sentence, the speaker is checking whether the fire extinguisher is in its place, so it is not being presented as a confirmed fact yet. That is why sé is very natural here.

Compare:

• Slökkvitækið er á sínum stað. = The fire extinguisher is in its place.
  This states a fact.
• Ég athuga hvort slökkvitækið sé á sínum stað. = I check whether the fire extinguisher is in its place.
  This treats it as something not yet confirmed.

slökkvitækið means the fire extinguisher.

It can be broken down like this:

• slökkvi- = related to slökkva, to extinguish / put out
• tæki = device, apparatus, tool
• -ið = the suffixed definite article, here meaning the

So:

• slökkvitæki = fire extinguisher
• slökkvitækið = the fire extinguisher

This is also a good reminder that Icelandic usually puts the at the end of the noun instead of using a separate word like English does.

Because gestirnir means the guests, while gestir just means guests.

• gestir = guests
• gestirnir = the guests

The ending -nir is the definite article attached to the noun. Icelandic often does this instead of using a separate word for the.

So:

• áður en gestir koma = before guests arrive
• áður en gestirnir koma = before the guests arrive

á sínum stað is an idiomatic expression meaning in its place or where it belongs.

Word by word:

• á = on / in / at
• sínum = its own / his own / her own / their own (depending on context)
• stað = place in the correct case form

So the whole phrase means something like:

• in its proper place
• where it should be

It is a very common expression in Icelandic.

Because sinn / sín / sitt is the reflexive possessive in Icelandic. It often means his/her/its/their own, referring back to the subject of the clause.

In hvort slökkvitækið sé á sínum stað, the subject of that clause is slökkvitækið, so sínum refers back to the fire extinguisher itself:

• á sínum stað = in its own place

This is more natural than using þess here.

A rough contrast:

• sinn/sín/sitt = refers back to the subject of the clause
• hans / hennar / þess / þeirra = refers to someone or something else

So sínum helps show that the place belongs to the fire extinguisher itself, not to some other thing.

Because the preposition á takes the dative when it describes location.

Here the meaning is static location: the extinguisher is in its place, not moving somewhere. So Icelandic uses the dative:

• á sínum stað

That is why:

• sínum is dative
• stað is also the dative form here

A very useful rule is:

• á + dative = location
• á + accusative = movement onto/into a place

So here it is dative because the extinguisher is being located, not moved.

Because Icelandic, like English, often uses the present tense in time clauses for future events.

So:

• áður en gestirnir koma literally uses present tense
• but in English we also normally say before the guests arrive, not before the guests will arrive

This is completely normal.

The same kind of thing happens in sentences like:

• Þegar hann kemur, förum við. = When he comes, we will leave.

So koma is present in form, but the time reference is future.

áður en means before when it introduces a full clause.

Here:

• áður en gestirnir koma = before the guests arrive

It is a set expression:

• áður = earlier / before
• en = introduces the following clause

So you can think of áður en as one unit meaning before.

Yes, the word order is normal.

The sentence has:

• a main clause: Ég athuga alltaf ...
• a hvort clause: hvort slökkvitækið sé á sínum stað
• a time clause: áður en gestirnir koma

In the two subordinate clauses, Icelandic uses normal subordinate-clause order:

• hvort slökkvitækið sé ...
• áður en gestirnir koma

That means the subject comes before the verb there.

So the structure is very standard and natural.

athuga usually means check, examine, look into, or make sure about depending on context.

In this sentence, check is probably the most natural translation:

• Ég athuga alltaf hvort ... = I always check whether ...

It does not necessarily mean a long, careful inspection. It can simply mean verifying that something is as it should be.

So the overall tone is practical and routine: the speaker regularly makes sure the fire extinguisher is where it should be before guests arrive.