Hún athugar aftur hvort heimilisfang móttakandans sé rétt og hvort símanúmer sendandans sé á pakkanum.

Here hvort means whether.

It introduces an indirect yes/no question:

• hvort heimilisfang móttakandans sé rétt = whether the recipient’s address is correct
• hvort símanúmer sendandans sé á pakkanum = whether the sender’s phone number is on the package

A very common learner question is whether hvort can mean if. In English, if and whether can overlap, but in Icelandic hvort is the standard choice in this kind of indirect question.

Because there are two separate whether-clauses:

• hvort heimilisfang móttakandans sé rétt
• og hvort símanúmer sendandans sé á pakkanum

In English, you might sometimes say She checks whether the address is correct and the phone number is on the package, without repeating whether. Icelandic often keeps the second hvort to make the structure clear.

Sé is the present subjunctive of vera (to be).
Er is the present indicative.

The subjunctive is common in subordinate clauses like this when something is being checked, questioned, or presented as uncertain rather than stated as a fact. Since she is checking whether something is true, Icelandic very naturally uses sé.

So:

• hvort ... sé rétt = whether ... is correct
• hvort ... sé á pakkanum = whether ... is on the package

A learner can think of it like this: the sentence is not declaring that these things are true; it is reporting that someone is verifying them.

They are in the genitive singular definite form.

They mean:

• móttakandans = the recipient’s
• sendandans = the sender’s

So:

• heimilisfang móttakandans = the recipient’s address
• símanúmer sendandans = the sender’s phone number

Icelandic often uses the genitive where English uses ’s.

Because a very normal Icelandic pattern is:

• noun + genitive possessor

So Icelandic often says the equivalent of:

• address of the recipient
• phone number of the sender

That is why you get:

• heimilisfang móttakandans
• símanúmer sendandans

This is one of the most basic differences between English and Icelandic noun phrases.

Even though English says the recipient’s address and the sender’s phone number, Icelandic often leaves the main noun without the definite article when it is followed by a definite genitive possessor.

So:

• heimilisfang móttakandans naturally means the recipient’s address
• símanúmer sendandans naturally means the sender’s phone number

The possessor phrase already makes the whole expression specific enough in meaning.

Because rétt is the neuter singular form of the adjective réttur (correct, right).

The noun heimilisfang is neuter singular, and predicate adjectives agree with the noun they describe. So:

• heimilisfang = neuter singular
• therefore rétt = neuter singular

If the noun were masculine or feminine, the adjective form would be different.

Because á can take different cases depending on meaning:

• accusative for motion onto something
• dative for location on something

Here the meaning is location: the phone number is on the package, not goes onto the package. So Icelandic uses the dative:

• á pakkanum = on the package

And pakkanum is the dative singular definite form of pakki.

Because Icelandic main clauses normally follow the verb-second pattern.

Here the first element is the subject:

• Hún = first element

So the finite verb comes second:

• athugar = second element

Then other sentence parts follow:

• aftur

So the order is:

• Hún athugar aftur ...

This is very natural Icelandic word order. English learners often expect adverbs to work exactly like in English, but Icelandic word order is more strongly shaped by the verb-second rule.

Normally, hvort is the better choice here.

Use hvort for whether in indirect yes/no questions, especially after verbs like:

• athuga = check
• vita = know
• spyrja = ask
• sjá = see

So athuga hvort ... = check whether ...

Ef is mainly if in conditional sentences, like if it rains. In some informal contexts people may use ef where standard grammar prefers hvort, but in this sentence hvort is the natural and correct choice.