Hún var að skera epli þegar síminn hringdi.

She was cutting an apple when the phone rang.

Breakdown of Hún var að skera epli þegar síminn hringdi.

• hún: she
• þegar: when
• hringja: to ring
• síminn: the phone
• eplið: the apple
• vera að skera: to be cutting

Questions & Answers about Hún var að skera epli þegar síminn hringdi.

Why does it say Hún and not Hana or Hennar?

Hún is the nominative form of hún (she) and it’s used for the subject of the sentence—the person doing the action.

• Hana is accusative (her) and would be used as a direct object (e.g., Ég sé hana = I see her).
• Hennar is genitive (her/hers) and marks possession or certain genitive uses (e.g., bíllinn hennar = her car).

What does the pattern var að + infinitive mean grammatically?

var að skera is a very common Icelandic way to express an action that was in progress at a certain time in the past—similar to English was cutting.

• var = past tense of vera (to be)
• að
  • skera (infinitive) = “to cut” as part of the construction
    So Hún var að skera epli corresponds closely to She was cutting an apple / She was in the middle of cutting an apple.

Is að here the infinitive marker “to,” and can it be omitted?

Yes: að is the normal infinitive marker (like English “to”). In the specific construction vera að + infinitive, að is generally required.
You can express a similar idea in other ways, but not usually by simply dropping að.

Why is it skera and not a conjugated form like sker or skar?

Because after að you normally use the infinitive. The finite (conjugated) verb in this clause is var.
So the grammar is: subject + var + að + infinitive.

What case is epli, and why doesn’t it change form?

epli is the direct object of skera, so it’s in the accusative. For many neuter nouns in Icelandic, nominative and accusative singular look the same, and epli is one of them.
So it is accusative here, it just doesn’t show a visible change.

Does epli mean “an apple” or “apples”? How do I know?

In this sentence it’s most naturally understood as an apple (singular), but Icelandic can sometimes leave number/definiteness more “bare” than English.

• epli can be singular (“an apple”) or also used more generically depending on context.
  If you want to be explicit:
• eplið = the apple
• epli (plural) typically appears as epli too (same form), so context helps; you can clarify with numbers (tvö epli = two apples).

Why is it þegar and not another word for “when”?

þegar is the standard conjunction for when introducing a time clause, especially in straightforward narration.
It commonly pairs with this kind of meaning: an ongoing action + a sudden event:
Hún var að skera epli þegar síminn hringdi.

Why does the second clause use simple past hringdi instead of something like “was ringing”?

Because the phone ringing is treated as a single event that interrupts the ongoing action. Icelandic often uses simple past for that “interrupting” event: hringdi = rang (once/at that moment).
If you wanted to emphasize continuous ringing, you could use other phrasing, but the common narrative pattern is:

• ongoing background: var að + infinitive
• interrupting event: simple past

What is síminn exactly—why the ending -inn?

síminn means the phone. The -inn is the definite article suffix attached to the noun:

• sími = a phone
• síminn = the phone
  It’s also masculine nominative singular, which fits because síminn is the subject of hringdi.

Why is the word order … epli þegar síminn hringdi and not something like … þegar hringdi síminn?

After þegar, Icelandic typically uses normal clause order: subject + verb → síminn hringdi.
You can get verb-first order in some contexts (especially questions, or with certain fronting/V2 effects), but þegar síminn hringdi is the neutral, most common order here.

How do I pronounce some tricky parts like Hún, skera, and hringdi?

A rough guide (accent varies by speaker):

• Hún: like “hoon” (long ú)
• skera: roughly “SKEH-ra” (with a rolled/tapped r)
• hringdi: starts with hr- (an h sound before r), roughly “HRING-di”
  Also note ll and nn can affect preceding vowels in Icelandic generally, but in síminn you mainly want a clear long í: “see-min(n)”.

Could I replace var að skera with another past form like skerði or skar?

You can, but the meaning changes:

• Hún skar epli þegar síminn hringdi. = “She cut an apple when the phone rang” (more like a completed action, less emphasis on “in progress”).
• Hún var að skera epli … = emphasizes the action was ongoing when interrupted.
  So var að + infinitive is the best match for the “was cutting” sense.