The u-umlaut page covered the productive, predictable rule — a stem a rounds to ö before a following u. This page is about a different, non-productive alternation: a small set of very common nouns that change their stem vowel in the plural by an old i-umlaut, plus a couple of outright suppletive plurals where the word is simply replaced. Fótur "foot" pluralises as fætur; bók "book" as bækur; maður "man/person" as menn. These are not rules you can apply to new words — they are lexicalised and must be memorised. But do not file them under "advanced curiosities." They are among the most frequent nouns in the entire language — foot, hand, book, mother, daughter, night, brother, man — so a learner meets them on day one and should drill them at A2/B1, not put them off to C1.
The vowel outcomes are a small, repeating set
Although each word must be learned individually, the vowel changes themselves cluster. The old i-umlaut (a fronting caused by an i in the historical plural ending, long since worn away) produces just a handful of outcomes:
| Singular vowel | Becomes (plural) | Example |
|---|---|---|
| a | e | maður → menn (with suppletion) |
| ó | æ | fótur → fætur, bók → bækur |
| ó (kinship -ir) | æ | móðir → mæður, dóttir → dætur, bróðir → bræður |
| o | e | bóndi → bændur (á/ó-grade), nótt → nætur |
| u | y | (the generic i-umlaut of u; rare in this noun set) |
The single most useful generalisation: ó and á front to æ (fótur → fætur, bók → bækur, bróðir → bræður, nótt → nætur), and a fronts to e (maður → menn). If you know that, most of the irregular plurals below are half-learned already, because the direction of the change is no longer a surprise — only the individual word membership is.
Body parts: fótur → fætur, hönd → hendur
Two of the highest-frequency body parts have irregular plurals. Fótur ("foot/leg") fronts ó → æ: fætur. Hönd ("hand") fronts to hendur (the singular hönd itself is already a u-umlauted form of an older hand-, which is why the plural surfaces with e). Here is the full declension of fótur, because it shows something important — the umlaut is in the plural, but the dative singular also umlauts:
| Case | Singular | Plural |
|---|---|---|
| Nefnifall (nom.) | fótur | fætur |
| Þolfall (acc.) | fót | fætur |
| Þágufall (dat.) | fæti | fótum |
| Eignarfall (gen.) | fótar | fóta |
Read the singular carefully: nominative fótur, accusative fót, genitive fótar all keep the ó — but the dative singular fæti umlauts, because that ending historically carried the triggering i. So the ó is kept in three of the four singular cases and fronts in the dative singular and across the plural nominative/accusative. (The dative plural fótum and genitive plural fóta keep the ó, since their endings had no i.) This is a genuinely tricky distribution and worth slowing down for.
Mig verkjar í fæturna eftir gönguna.
My feet hurt after the walk. Accusative plural 'fæturna' (fætur + article) — the ó has fronted to æ.
Hún steig á annan fótinn og hrasaði.
She stepped on one foot and stumbled. Accusative singular 'fótinn' (fót + article) — singular keeps the ó.
Það er sár á öðrum fæti.
There's a sore on one foot. Dative SINGULAR 'fæti' — note the dative singular also umlauts, unlike the other singular cases.
Ég get ekki staðið á fótunum.
I can't stand on my feet. Dative plural 'fótunum' — back to ó, because the dative-plural ending had no triggering i.
Hann er með stórar hendur.
He has big hands. Plural 'hendur' from 'hönd' — the umlauted plural.
Kinship: móðir → mæður, dóttir → dætur, bróðir → bræður
The family terms in -ir form a tight cluster: they front ó → æ in the plural and take the ending -ur. Móðir "mother" → mæður; dóttir "daughter" → dætur; bróðir "brother" → bræður. (Faðir "father" → feður and systir "sister" → systur round out the family, with their own vowels.) These nouns also have slightly irregular singulars worth a glance — móðir, móður, móður, móður — but the headline you must own is the umlauted plural:
| Singular | Plural (nom.) | Gloss | Change |
|---|---|---|---|
| móðir | mæður | mother(s) | ó → æ |
| dóttir | dætur | daughter(s) | ó → æ |
| bróðir | bræður | brother(s) | ó → æ |
| faðir | feður | father(s) | a → e |
| systir | systur | sister(s) | (no umlaut) |
Mæður þeirra þekkjast frá því í skóla.
Their mothers have known each other since school. Plural 'mæður' from 'móðir' — ó → æ.
Þau eiga þrjár dætur og einn son.
They have three daughters and one son. Plural 'dætur' from 'dóttir' — ó → æ.
Bræður mínir búa allir erlendis.
My brothers all live abroad. Plural 'bræður' from 'bróðir' — ó → æ.
Time words: nótt → nætur, vetur → vetur
The calendar gives two more. Nótt ("night") fronts ó → æ: nætur. And vetur ("winter") is the odd one out — its plural is identical to its singular: einn vetur "one winter", margir vetur "many winters". No change at all. It's worth knowing precisely because it breaks the pattern you'd expect.
| Singular | Plural | Gloss |
|---|---|---|
| nótt | nætur | night → nights (ó → æ) |
| vetur | vetur | winter → winters (no change) |
Við gistum þrjár nætur á hótelinu.
We stayed three nights at the hotel. Plural 'nætur' from 'nótt' — ó → æ.
Hann hefur búið hér í tuttugu vetur.
He has lived here for twenty winters. Plural 'vetur' — identical to the singular; no umlaut.
Suppletion: maður → menn, and the -ndi nouns
A few plurals are not umlaut at all but suppletion — a different stem. The star case is maður ("man, person, one"), whose plural is menn ("men, people"). This is one of the ten most common nouns in Icelandic, so its irregular plural is unavoidable from the first week.
| Case | Singular | Plural |
|---|---|---|
| Nom. | maður | menn |
| Acc. | mann | menn |
| Dat. | manni | mönnum |
| Gen. | manns | manna |
Two nouns ending in -ndi also take an umlauted -ur plural with ó/á → æ: bóndi ("farmer") → bændur, and frændi ("male relative") → frændur (no umlaut here). Bóndi → bændur is the one to lock in, since it patterns visibly like bróðir → bræður.
Það voru margir menn á fundinum.
There were many people at the meeting. Plural 'menn' from 'maður' — suppletive, not umlaut.
Ég talaði við tvo menn úti á götu.
I spoke to two men out on the street. Accusative plural 'menn'.
Bændur í dalnum rækta sauðfé.
Farmers in the valley raise sheep. Plural 'bændur' from 'bóndi' — ó → æ.
Það er gott að búa innan um gott fólk og gott fólk gerir góða menn.
It's good to live among good people, and good people make good men. Two plurals here: 'fólk' is grammatically singular (a collective), but 'menn' is the true plural of 'maður'.
Why you must memorise these (and how to)
There is no shortcut that turns these into a rule — the i-umlaut that created them stopped being productive a thousand years ago, so a learner cannot derive fætur from fótur by any live process, the way you can derive löndum from land. You simply have to know that this set of nouns umlauts. The honest advice is to memorise them in their semantic clusters — body parts (fótur/fætur, hönd/hendur), kinship (móðir/mæður, dóttir/dætur, bróðir/bræður), time (nótt/nætur, vetur/vetur), people (maður/menn, bóndi/bændur) — because that is how they come up in real sentences and because the shared vowel outcome (mostly ó/á → æ) makes the cluster cohere. The other half of the discipline is remembering that the umlaut is in the plural (and the dative singular of fótur), while the rest of the singular keeps the original vowel.
Common Mistakes
❌ Ég er með kalda fótar.
Incorrect — the plural of 'fótur' is 'fætur', not a regularised '-ar' plural. (And the case is wrong too.)
✅ Ég er með kalda fætur.
My feet are cold. Plural 'fætur' — ó → æ.
❌ Hann á tvær dótturir.
Incorrect — 'dóttir' has the suppletive/umlauted plural 'dætur'; you cannot add a regular ending.
✅ Hann á tvær dætur.
He has two daughters. Plural 'dætur' — ó → æ.
❌ Það voru margir maðurar þarna.
Incorrect — the plural of 'maður' is the suppletive 'menn', never a regularised '*maðurar'.
✅ Það voru margir menn þarna.
There were many people there. Plural 'menn'.
❌ Ég borða þrjú epli á dag og einn appelsínu. — wait, the trap: 'á fæti' written 'á fóti' for the dative singular.
Incorrect — the DATIVE SINGULAR of 'fótur' also umlauts: 'fæti', not '*fóti'. The singular umlauts only in the dative.
✅ Það er bólga á fæti.
There's a swelling on one foot. Dative singular 'fæti' — umlauted.
❌ Umlauting the whole singular: 'ég sá fæt' for 'I saw a foot'.
Incorrect — the accusative SINGULAR keeps the ó: 'fót'. The umlaut is plural-only (plus the dat.sg 'fæti'), not across the singular.
✅ Ég sá fót undir teppinu.
I saw a foot under the blanket. Accusative singular 'fót' — keeps the ó.
Key Takeaways
- A small set of very common nouns form their plural by old i-umlaut (non-productive) or by suppletion — memorise them; you cannot derive them.
- The vowel outcomes cluster: ó/á → æ (fótur → fætur, bók → bækur, móðir → mæður, dóttir → dætur, bróðir → bræður, nótt → nætur, bóndi → bændur) and a → e (maður → menn, faðir → feður).
- The umlaut sits in the plural — the singular nominative/accusative/genitive keep the original vowel. The one singular exception is the dative singular of fótur: fæti.
- Vetur doesn't change at all (sg = pl); maður → menn and bóndi → bændur are suppletive/irregular and unavoidable.
- Drill these in semantic clusters (body parts, kinship, time, people) at A2/B1 — they are everyday words, not a C-level luxury.
Now practice Icelandic
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Start learning Icelandic→Related Topics
- u-Umlaut in Plurals and the Dative PluralA2 — The single most pervasive sound rule in Icelandic noun inflection: a stem 'a' rounds to 'ö' before a following 'u' — most reliably in the dative-plural ending -um (dögum, löndum) and in many bare plurals (barn → börn, land → lönd).
- Strong Feminine: u-umlaut Monosyllables (bók, hönd, mörk)B1 — The high-frequency strong feminine monosyllables whose plurals change the stem vowel dramatically — bók → bækur, hönd → hendur, mörk → merkur, nótt → nætur — plus the kinship irregulars móðir → mæður and dóttir → dætur, with full paradigms showing how hönd cycles through three stem vowels.
- Making Plurals: The BasicsA1 — An A1 orientation to Icelandic noun plurals — they depend on gender (masc -ar/-ir, fem -ur/-ir, neuter often no ending at all), some change their vowel (bók→bækur, barn→börn), and there is no -s plural anywhere in the language.
- I-Umlaut as a Sound AlternationB1 — I-umlaut (i-hljóðvarp) is an older fronting alternation frozen into Icelandic paradigms: a lost i or j in the next syllable pulled the stem vowel forward — a→e, o→y, u→y, á/ó→æ, ú→ý, au→ey. It explains maður→menn, fótur→fætur, stór→stærri, ungur→yngri. Unlike u-umlaut it is no longer productive, so you memorise the affected sets — but the same alternation links surprising word-families.
- The Four Cases and What They DoA1 — A functional introduction to Icelandic's four cases — nefnifall, þolfall, þágufall, eignarfall — focused on the jobs each one does and the crucial fact that case is assigned by verbs and prepositions, not chosen freely or fixed by word position.