Reykskynjarinn pípir stundum þegar hún eldar á miklum hita.

Because -inn is the suffixed definite article in Icelandic.
So:

• reykskynjari = smoke detector
• reykskynjarinn = the smoke detector

Icelandic usually attaches the to the end of the noun instead of putting a separate word before it.

Also, Reykskynjarinn is a compound word:

• reykur = smoke
• skynjari = sensor / detector

So the whole word literally means something like the smoke-detector.

Pípir is the 3rd person singular present tense of að pípa.

So:

• að pípa = to beep / chirp / make a peeping sound
• pípir = beeps

It matches the subject Reykskynjarinn, which is singular:

• Reykskynjarinn pípir = The smoke detector beeps

The present tense here describes something that happens generally or repeatedly, not just right now.

So Reykskynjarinn pípir stundum... means something like:

• The smoke detector sometimes beeps...
• The smoke detector will sometimes beep...

This is a very normal use of the Icelandic present tense for habits and repeated situations.

Stundum means sometimes.

In this sentence it comes after the verb:

• Reykskynjarinn pípir stundum...

That is a very natural placement. Icelandic adverbs like stundum can sometimes move around depending on emphasis, but this version is neutral and common.

For example, you could also say:

• Stundum pípir reykskynjarinn...

But then the sentence has a slightly different focus, with sometimes emphasized more.

Here þegar means when and introduces a time clause:

• þegar hún eldar á miklum hita = when she cooks on high heat

A very common learner issue is that þegar can also mean already in other contexts. But in this sentence, it is definitely the conjunction when.

Because þegar starts a subordinate clause. In this kind of clause, Icelandic normally has the subject before the verb:

• þegar hún eldar = when she cooks

This is different from the main-clause verb-second pattern that Icelandic often uses.

So:

• main clause: Reykskynjarinn pípir stundum
• subordinate clause: þegar hún eldar á miklum hita

Eldar is the 3rd person singular present tense of að elda.

So:

• að elda = to cook
• hún eldar = she cooks / she is cooking

Just like in English, the exact translation depends on context. Here it has a habitual meaning: when she cooks on high heat.

In this sentence, hún most naturally means she, referring to a female person.

However, Icelandic pronouns can also refer back to nouns by grammatical gender. So in other contexts, hún might refer to a feminine noun, not necessarily a female person.

But if this sentence is given on its own, most learners should understand hún as she.

It means on high heat or at high heat.

This is an idiomatic cooking expression. It does not mean something overly literal like on much heat. The idea is that the cooking is happening with strong / intense heat.

So the whole clause:

• þegar hún eldar á miklum hita

means:

• when she cooks on high heat

Because the preposition á takes the dative here.

In this sentence, á describes a state or setting rather than movement, so Icelandic uses the dative:

• á miklum hita

Here:

• hiti is the noun meaning heat
• its dative singular form is hita
• mikill = great / much / strong
• its dative singular masculine form is miklum

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

• miklum hita = high / intense heat in the dative

Because hiti is the nominative form, but after á in this expression, Icelandic uses the dative.

So:

• nominative: hiti
• dative: hita

That is why the sentence has á miklum hita, not á mikill hiti or á miklum hiti.

Yes. Even if the basic meaning stays the same, natural English can vary a bit:

• The smoke detector sometimes beeps when she cooks on high heat.
• The smoke detector will sometimes beep when she cooks on high heat.
• The smoke alarm sometimes goes off when she cooks on high heat.

The Icelandic sentence itself is straightforward, but English may choose beeps, chirps, or goes off depending on style and context.