Learn Hardcore Icelandic: Ég þarf kennitöluna þína.

Ég — first-person singular nominative pronoun: I.
þarf — present tense, 1st person singular of þurfa: need.
kennitöluna — the noun kennitala (ID number), in accusative singular, definite: “the ID number.” It shows vowel change (a → ö) and takes the definite ending -na.
þína — possessive your (singular, informal), agreeing with a feminine accusative singular noun: your.

Why is kennitöluna in the accusative and definite?

Accusative: þurfa governs the accusative for the thing needed, so the direct object is in the accusative.
Definite: When a possessive pronoun is placed after the noun, the noun is typically made definite with the suffixed article. Hence: kennitöluna þína (“the ID number your” = “your ID number”).

Why does kennitala become kennitöluna (with ö and -na)?

It’s a combination of case and definiteness:

Base noun: kennitala (fem., “ID number”).
Accusative singular of this noun is kennitölu. The a → ö change is a regular sound change called u-umlaut, triggered by a following u in the ending.
Definite accusative adds the article -na: kennitöluna.

Why is the possessive þína after the noun, and could it go before?

Postposed possessive (after the noun) is very common and requires the noun to be definite: kennitöluna þína. This is the neutral, typical wording.
Preposed possessive is also possible and makes the noun indefinite: þína kennitölu. This can sound a bit more emphatic or contrastive (your number, not someone else’s), but both forms are fine in many contexts.

Can I say “Ég þarf kennitala þína” (without the definite ending)?

No—not with the possessive after the noun. A postposed possessive normally demands a definite noun. Use either:

Why is it þína and not þín?

The possessive pronoun agrees with the noun in gender, number, and case:

Does þurfa always take an object like this? When do I use að?

With a noun phrase direct object: Ég þarf kennitöluna þína.
With an action you need to do, use að
- infinitive: Ég þarf að fá kennitöluna þína (“I need to get your ID number”), Ég þarf að hringja (“I need to call”).

Is “Ég þarf…” too blunt? How do I sound more polite?

“Ég þarf …” can be a bit direct in service contexts. Softer options:

Má ég fá kennitöluna þína? (May I get your ID number?)
Get ég fengið kennitöluna þína? (Can I get your ID number?)
Gætirðu gefið mér kennitöluna þína? (Could you give me your ID number?) You can add vinsamlegast (please) to be extra polite.

Could I use vantar instead of þarf?

Yes, but the structure changes:

Mig vantar kennitöluna þína. (Literally “Me lacks your ID number.”) Here, the person who lacks something is in the accusative (mig), and the thing lacking is also accusative. Both are idiomatic: Ég þarf kennitöluna þína and Mig vantar kennitöluna þína.

How would I say “your” when speaking to more than one person?

Use ykkar (plural “your”), which does not change for case/gender:

Ég þarf kennitöluna ykkar. (I need your [pl.] ID number.) A very formal (and rare) possessive is yðar: Ég þarf kennitöluna yðar.

Pronunciation tips for the tricky letters and vowels?

Ég: the g is soft; it often sounds like a light y/gh; roughly “yeh.”
þ in þarf/þína: unvoiced th, like English “think,” not “this.”
Double nn in kenni is a short, crisp n.
ö in tölu is a rounded vowel (similar to British “nurse” but with rounded lips).
Stress is on the first syllable of each word and on the first part of compounds: KEN-ni-tö-lu-na.

Can I drop þína if context makes it obvious?

Yes. If it’s clear whose number is meant, you can say:

Ég þarf kennitöluna. (I need the ID number.) Including þína simply disambiguates and can be more natural in first contact.

How does kennitala decline in the most common forms?

Singular:

How do I say this for other persons? (my/his/her own)

My: Ég þarf kennitöluna mína.
Your (sg.): Ég þarf kennitöluna þína.
His/her (someone else’s): Hann þarf kennitöluna hans / Hún þarf kennitöluna hennar.
His/her own (reflexive, same subject): Hann/Hún þarf kennitöluna sína. Use sína only when the owner is the sentence’s subject.

Can I change the word order for emphasis?

Yes. Icelandic allows fronting for emphasis:

Neutral: Ég þarf kennitöluna þína.
Emphatic object: Kennitöluna þína þarf ég. The meaning stays the same; the emphasis changes.

How is þurfa conjugated in the present?

Ég þarf
Þú þarft
Hann/Hún/Það þarf
Við þurfum
Þið þurfið
Þeir/Þær/Þau þurfa Past for reference: ég þurfti, þú þurftir, hann þurfti, við þurftum, þið þurftuð, þeir þurftu.