The numbers overview makes the headline point: only 1, 2, 3 and 4 inflect, and they stop dead at four. This page delivers the detail that overview promised — the complete declension of all four, across three genders and four cases. That sounds like a lot of forms, but two facts make it manageable: einn declines like a perfectly ordinary strong adjective, and tveir/þrír/fjórir share one tidy pattern (same dative -um/-ur, same genitive in doubled consonants). Master these and you can count anything, in any case a preposition throws at you.
einn — declines like a strong adjective
einn ("one") behaves exactly like the strong adjective gamall. There is nothing exotic here; if you know the strong adjective endings, you already know einn. Because "one" is by definition singular, there is no plural (except in the special sense "alone / some," which you can set aside for now).
| Case | Masculine (kk) | Feminine (kvk) | Neuter (hk) |
|---|---|---|---|
| Nominative | einn | ein | eitt |
| Accusative | einn | eina | eitt |
| Dative | einum | einni | einu |
| Genitive | eins | einnar | eins |
einn maður, ein kona, eitt barn
one man (kk), one woman (kvk), one child (hk) — three nominative forms of 'one'.
Ég talaði við einn mann og eina konu.
I spoke to one man and one woman. (accusative: einn / eina)
Ég kom með einum vini.
I came with one friend. (dative after 'með': einum)
tveir, þrír, fjórir — one shared pattern
The numbers "two," "three" and "four" exist only in the plural (you can't have a singular "two"), and they share a single elegant pattern. Learn the shape once and it covers all three: the dative ends in -ur/-um and the genitive has a doubled consonant (tveggja, þriggja, fjögurra). Here are all three laid out together so the parallels jump out.
tveir (two)
| Case | Masculine (kk) | Feminine (kvk) | Neuter (hk) |
|---|---|---|---|
| Nominative | tveir | tvær | tvö |
| Accusative | tvo | tvær | tvö |
| Dative | tveimur | tveimur | tveimur |
| Genitive | tveggja | tveggja | tveggja |
þrír (three)
| Case | Masculine (kk) | Feminine (kvk) | Neuter (hk) |
|---|---|---|---|
| Nominative | þrír | þrjár | þrjú |
| Accusative | þrjá | þrjár | þrjú |
| Dative | þremur | þremur | þremur |
| Genitive | þriggja | þriggja | þriggja |
fjórir (four)
| Case | Masculine (kk) | Feminine (kvk) | Neuter (hk) |
|---|---|---|---|
| Nominative | fjórir | fjórar | fjögur |
| Accusative | fjóra | fjórar | fjögur |
| Dative | fjórum | fjórum | fjórum |
| Genitive | fjögurra | fjögurra | fjögurra |
Two structural gifts hide in these tables. First, the dative and genitive are gender-blind: tveimur covers all three genders, and so do þremur, fjórum, tveggja, þriggja, fjögurra. Gender only matters in the nominative and accusative. Second, the dat/gen forms are the same whether you're counting men, books or children — so once a preposition forces the dative, you can stop worrying about gender.
The three genders in the nominative
This is the level most learners need first: pick the right "two/three/four" for the gender of the thing you're counting.
tveir hestar, tvær bækur, tvö hús
two horses (kk), two books (kvk), two houses (hk).
þrír strákar, þrjár stelpur, þrjú epli
three boys (kk), three girls (kvk), three apples (hk).
fjórir bílar, fjórar krónur, fjögur ár
four cars (kk), four krónur (kvk), four years (hk).
The dative: forced by prepositions
Icelandic prepositions govern case, and many common ones (með "with," frá "from," af "off/of," að "to") take the dative. When you count a dative noun, the number goes dative too. This is where tveimur, þremur, fjórum earn their keep.
Ég kom með tveimur hundum.
I came with two dogs. (með + dative → tveimur, gender-blind)
Hún býr með þremur vinkonum.
She lives with three (female) friends. (þremur)
Bíllinn er frá fjórum mismunandi eigendum.
The car has had four different owners. (frá + dative → fjórum)
The genitive: compounds and partitives
Here is the pattern competitors leave out — and the one that makes you sound genuinely fluent. The genitive of these numbers (tveggja, þriggja, fjögurra) is everywhere in Icelandic compounds and measurements: a flat is described by how many rooms it has, a child's age by how many years, a parent by how many children. The number sits in the genitive in front of a genitive-plural noun.
Hún er þriggja barna móðir.
She's a mother of three children. (þriggja barna — genitive)
Við leigjum þriggja herbergja íbúð.
We're renting a three-room flat. (þriggja herbergja — the standard way Icelanders describe a flat's size)
Þetta er tveggja ára gamall bíll.
This is a two-year-old car. (tveggja ára — age in the genitive)
Once you notice this, you'll see it on every property listing and in every introduction: fjögurra herbergja íbúð (four-room flat), tveggja vikna frí (a two-week holiday). It is not advanced — it is daily life. The forms to lock in are the doubled-consonant genitives: tveggja, þriggja, and fjögurra (note the -rr-).
Common Mistakes
❌ tvö bækur
Incorrect — bók is feminine, so 'two' is tvær: tvær bækur.
✅ tvær bækur
two books
❌ Ég kom með tveir hundum.
Incorrect — 'með' takes the dative; 'two' must be tveimur, not the nominative tveir.
✅ Ég kom með tveimur hundum.
I came with two dogs.
❌ þriggja herbergja íbúð → þrír herbergja íbúð
Incorrect — the compound needs the genitive þriggja, not the nominative þrír.
✅ þriggja herbergja íbúð
a three-room flat
❌ fjögura ára
Incorrect spelling — the genitive of 'four' is fjögurra, with -rr-.
✅ fjögurra ára
four years (gen.) / four-year-old
❌ Ég sá þrír stelpur.
Incorrect — stelpa is feminine, so accusative 'three' is þrjár: ég sá þrjár stelpur.
✅ Ég sá þrjár stelpur.
I saw three girls.
Key Takeaways
- einn declines like a strong adjective: einn/ein/eitt, dat einum/einni/einu, gen eins/einnar/eins. Watch the doubled consonants in eitt, einni, einnar.
- tveir, þrír, fjórir are plural-only and share one pattern: gender shows only in nom/acc; the dative (tveimur, þremur, fjórum) and genitive (tveggja, þriggja, fjögurra) are gender-blind.
- Nominative gender forms: kk tveir/þrír/fjórir, kvk tvær/þrjár/fjórar, hk tvö/þrjú/fjögur. Mind the ö in tvö, fjögur.
- Prepositions like með/frá/af force the dative — use tveimur/þremur/fjórum regardless of gender.
- The genitive (tveggja/þriggja/fjögurra) powers everyday compounds: þriggja herbergja íbúð, tveggja ára gamall. Note the spellings tveggja, þriggja, fjögurra (-rr-).
Now practice Icelandic
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Start learning Icelandic→Related Topics
- Numbers: Why 1-4 Are SpecialA1 — A map of the Icelandic number system built around its most surprising feature: the numerals 1 to 4 decline for gender and case (einn/ein/eitt, tveir/tvær/tvö ...), while 5 and above are normally invariant — a clear, learnable boundary.
- Cardinals 5 and Above, Hundreds and ThousandsA2 — From fimm upward the cardinals are essentially invariant (fimm, sex, sjö … tuttugu, þrjátíu), joined by og in compounds — but the catch English speakers miss is that a compound ending in 1-4 still re-inflects that last element for gender (þrjátíu og tvær bækur, hundrað tuttugu og ein bók), and hundrað/þúsund are neuter nouns that pluralise (tvö hundruð).
- Telling Time and DatesA2 — How to tell the clock and say the date in Icelandic — klukkan er þrjú, the half-hour trap (hálf níu = 8:30, counting UP to the next hour like German), korter yfir/í for quarters, the 24-hour clock, and dates built on ordinals (fjórði júní, þann fimmta).