Hún notar öryggishnappinn þegar hún dettur.

Icelandic very often makes compound nouns, where English might use two separate words.

So:

• öryggi = safety, security
• hnappur = button

Together they become öryggishnappur = safety button / emergency button.

This is completely normal in Icelandic. English learners often want to separate words like this, but in Icelandic compounds are usually written as a single word.

The first part of the compound comes from öryggi, but it appears in the form öryggis-.

That -s is very common in Icelandic compounds. Historically and grammatically, it often reflects a genitive form or a linking form. So:

• öryggi = safety
• öryggis- = safety- / of safety, used in compounds

So öryggishnappur is literally something like button of safety, but in natural English you would just think of it as safety button or emergency button.

The ending -inn is the definite article, meaning the.

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

So:

• öryggishnappur = a safety button
• öryggishnappinn = the safety button

In this sentence, it is also in the accusative singular, which is why the exact form is -inn here.

Because it is the direct object of the verb notar.

The verb nota means to use, and it normally takes an accusative object. So:

• Hún = subject, nominative
• notar = uses
• öryggishnappinn = direct object, accusative

A useful way to think about it is: she is using something, and that something goes into the accusative.

Because the sentence has two clauses, and each clause needs its own subject.

• Main clause: Hún notar öryggishnappinn
• Subordinate clause: þegar hún dettur

English also usually repeats the subject here: She uses the safety button when she falls. Icelandic does the same thing.

So even though it is the same person in both clauses, you still say hún again.

Detta is the infinitive, meaning to fall.

But in the sentence you need a finite verb that matches hún = she, so you use the 3rd person singular present form:

• detta = to fall
• dettur = falls / is falling

So:

• hún dettur = she falls

This is a normal verb change. Icelandic verbs must be conjugated for person and tense.

Þegar means when and introduces a subordinate clause.

After þegar, the clause is:

• hún = subject
• dettur = verb

So the order is simply subject + verb.

That is different from some main-clause patterns in Icelandic, where the verb often comes in the second position. Here, because þegar starts a subordinate clause, hún dettur is the normal order.

Yes. That is a perfectly good sentence.

If you start with the þegar clause, the main clause changes word order slightly because Icelandic follows a verb-second pattern in main clauses.

So you get:

• Þegar hún dettur, notar hún öryggishnappinn.

Notice that after the fronted clause, the verb notar comes before the subject hún in the main clause.

Both versions are correct:

• Hún notar öryggishnappinn þegar hún dettur.
• Þegar hún dettur, notar hún öryggishnappinn.

They are related, and both can mean to fall, but they are not always used in exactly the same way.

• detta is very common for a person or object suddenly falling, dropping, or falling off
• falla is often broader and can sound a bit more formal or general in some contexts

In everyday speech, detta is very natural for physical falling like in this sentence.

So here, dettur is a very normal choice.