Learn Hardcore Icelandic: Ég skrifa símanúmerið og kennitöluna á pöntunina. - I write the phone number and the ID number on the reservation.

símanúmerið = símanúmer (phone number, neuter) + -ið → “the phone number.”
kennitöluna = kennitala (ID number, feminine) + accusative definite -na, with vowel change (see below) → “the ID number.”
pöntunina = pöntun (order, feminine) + accusative definite -ina → “the order.” Using these definite forms implies specific, known items (e.g., the customer’s phone and ID number on that particular order).

What case are the nouns in, and why?

All three are in the accusative singular:

símanúmerið and kennitöluna are direct objects of skrifa (to write), which typically takes its object in the accusative.
á pöntunina uses á (a two-way preposition). With motion/placement onto something, á takes the accusative; here you’re writing onto the order, so accusative (pöntunina).

Why does kennitala become kennitöluna?

That’s u-umlaut (vowel harmony): in many feminine -a nouns, an a in the stem becomes ö in oblique cases (accusative/dative/genitive) because of a historical u in the ending. So:

nominative: kennitala
accusative (indef.): kennitölu
accusative (def.): kennitöluna

Why á pöntunina and not á pöntuninni?

Because the action involves putting writing onto the order (direction/placement), which takes accusative after á: á pöntunina. If you’re describing location (no movement), you use dative: á pöntuninni.

Placement: Ég skrifa þetta á pöntunina.
Location/state: Textinn er á pöntuninni.

Can I use í instead of á here?

You can, but it changes the nuance:

á pöntunina = onto the order (on the paper/form).
í pöntunina = into the order (into the order record in a system/database). So if you mean entering data into an electronic order, í is natural, often with verbs like skrá or færa inn:
Ég skrái upplýsingarnar í pöntunina.

Could I say Ég skrifa niður …?

Yes. skrifa niður means “to write down.”

Without specifying where: Ég skrifa niður símanúmerið og kennitöluna.
If you still want the destination, you can say: Ég skrifa símanúmerið og kennitöluna niður á pöntunina (down onto the order). Both patterns occur; many speakers would simply use one or the other depending on context.

Is the word order fixed?

The neutral order is SVO + prepositional phrase: Ég skrifa X … á Y. You can front elements for emphasis:

Á pöntunina skrifa ég símanúmerið og kennitöluna. (Emphasis on the destination)
Símanúmerið og kennitöluna skrifa ég á pöntunina. (Emphasis on what is written) The basic relationships don’t change.

How do I say the progressive “I am writing …” in Icelandic?

Use vera + að + infinitive:

Ég er að skrifa símanúmerið og kennitöluna á pöntunina.

How do I pronounce the tricky parts?

Ég: initial sounds like English “ye-”; final g is a soft fricative; roughly “yegh.”
á: like the vowel in English “ow,” but a single Icelandic vowel.
ö (in pöntunina, kennitöluna): like French “eu” in “peu.”
ð (in númerið): like the “th” in “this.”
Double nn (in kennitala): a long consonant; hold the “n” slightly longer. Primary stress is on the first syllable of each word.

What are the genders of these nouns?

símanúmer (phone number): neuter.
kennitala (ID number): feminine.
pöntun (order): feminine. You’ll usually see these noted in dictionaries as n., f., etc. Gender affects endings and agreement.

What would the indefinite version look like?

All indefinite: Ég skrifa símanúmer og kennitölu á pöntun. (“a phone number,” “an ID number,” “an order”)
Mixed (very common): Ég skrifa símanúmer og kennitölu á pöntunina. (indefinite items on a specific order) Choose definite vs. indefinite according to what is specific/known in context.

Why isn’t ég capitalized like English “I”?

Icelandic doesn’t capitalize the first-person singular pronoun. It’s lowercase ég, except at the start of a sentence (where any word would be capitalized).

Does og (“and”) affect the case of the coordinated nouns?

No. Both coordinated nouns take the case required by the verb/preposition. Here, both símanúmerið and kennitöluna are accusative because skrifa takes an accusative object.

What exactly is a kennitala?

It’s the Icelandic national ID number (10 digits, typically written as DDMMYY-XXXX). It’s used widely for identification in Iceland, so kennitöluna here refers to that specific number.

Could I use other verbs like setja, skrá, or færa inn?

Yes, depending on nuance:

setja [X] á pöntunina = put/place X onto the order (physical or figurative placement).
skrá [X] í pöntunina = register/record X into the order (system/database).
færa [X] inn í pöntunina = enter X into the order (data entry). Choose the preposition accordingly: á for “onto,” í for “into.”

Can I drop the destination phrase (á pöntunina)?

If context makes the destination obvious, yes: Ég skrifa símanúmerið og kennitöluna. Otherwise, include the prepositional phrase to avoid ambiguity.