If you learn one sound rule in Icelandic, learn this one. A stem vowel a rounds to ö whenever a u follows it in the next syllable. This is the u-umlaut (u-hljóðvarp), and it is the most frequent vowel alternation in the entire language — it touches nouns, adjectives, and verbs alike. This page is about the rule as it shows up in noun plurals, where it has one near-universal trigger: the dative-plural ending -um. Master "see -um, round stem a to ö," and you will fix one of the most common learner errors across the whole noun system in a single stroke. (The pure pronunciation side of the rule — how ö actually sounds — lives on the u-umlaut sound page; the i-umlaut plurals like maður → menn are a different rule, on irregular plurals.)
The rule in one line
A stem a becomes ö when the following syllable contains u.
That u can be a vowel you still see (the -u of a feminine plural, the -u- of a dative plural) or a u that has since disappeared but left the rounding behind (this is why a bare plural like börn is rounded even though no u is visible — historically there was one). For a learner, the practical version is simpler: the dative-plural -um always rounds a stem a, and several plural forms do too.
Við bjóðum öllum börnunum í afmælið.
We're inviting all the children to the birthday party. 'börnunum' — the dative-plural -um rounds the stem 'a' of 'barn' to 'ö'.
Það eru margir ferðamenn í löndunum í kringum okkur.
There are many tourists in the countries around us. 'löndunum' — dative plural of 'land', stem a → ö.
It applies in every gender
The u-umlaut is not tied to one noun class — it is a sound rule, so it fires wherever the phonetic condition (stem a, following u) is met. Here is one noun per gender, each rounding in its plural forms:
| Gender | Singular (nom.) | Plural (nom.) | Dative plural | Rounding? |
|---|---|---|---|---|
| Masculine (kk) | dagur ("day") | dagar | dögum | only in dat.pl (the -um) |
| Feminine (kvk) | saga ("story") | sögur | sögum | in nom.pl AND dat.pl |
| Neuter (hk) | land ("land") | lönd | löndum | in nom.pl AND dat.pl |
Look closely at what differs. For the masculine dagur, the nominative plural dagar keeps its a (the ending is -ar, no u), but the dative plural dögum rounds, because -um has the u. For the feminine saga and neuter land, both the bare/nominative plural (sögur, lönd) and the dative plural (sögum, löndum) round — a-stem feminines and neuters round in the plural across the board.
Ég vinn alla daga vikunnar nema sunnudaga.
I work every day of the week except Sundays. 'daga' (acc.pl) keeps the a — no u in the ending.
Á þremur dögum gengum við yfir hálendið.
In three days we walked across the highlands. 'dögum' — dative plural, the -um rounds 'dag-' to 'dög-'.
Amma kann ótal sögur frá gamla tímanum.
Grandma knows countless stories from the old days. 'sögur' — feminine nominative/accusative plural rounds because the ending is -ur.
Hún hefur ferðast til margra landa, en býr enn á Íslandi.
She has travelled to many countries but still lives in Iceland. 'landa' (gen.pl) keeps the a — the ending is -a, no u.
Eldgos hafa mótað landslagið í þessum löndum um aldir.
Volcanic eruptions have shaped the landscape in these countries for centuries. 'löndum' — dative plural, stem rounded.
Notice that the genitive plurals — daga, landa — generally end in -a, not -u, so they do not round. This is the whole logic of the rule made visible in a single noun: löndum rounds (there's a u), landa does not (there's an a).
When there is no a, there is nothing to round
The rule only ever touches a stem a. If the stem vowel is anything else, the dative plural -um attaches with no change at all:
| Singular | Dative plural | Note |
|---|---|---|
| hestur ("horse") | hestum | stem 'e' — nothing to round |
| hús ("house") | húsum | stem 'ú' — nothing to round |
| fiskur ("fish") | fiskum | stem 'i' — nothing to round |
| land ("land") | löndum | stem 'a' — rounds to 'ö' |
Það eru margir hestar í dölunum hérna.
There are many horses in the valleys here. 'hestar' has no stem a to round; but 'dölunum' (from 'dalur', valley) does round — dal- → döl-.
That last example is a useful contrast in one sentence: hestar stays put (stem e), while dölunum rounds (stem a + the -um). Same sentence, same plural ending region, opposite outcome — decided purely by whether the stem vowel is an a.
Two a's in a row: the chain effect
A subtler case: when a word has two stem syllables that both contain a, the rounding can affect both — the classic illustration is barnabörn ("grandchildren," literally "children's children"). The first element barna- (genitive of barn) keeps its a, while the second element börn is the rounded plural of barn. Compounds therefore often show a in the first half and ö in the second:
Hún á sex barnabörn og tvö barnabarnabörn.
She has six grandchildren and two great-grandchildren. 'barna-' keeps a (genitive linking form); 'börn' rounds (plural).
Gömlu hjónin sögðu sögurnar sínar fyrir barnabörnunum.
The old couple told their stories to the grandchildren. 'barnabörnunum' — note the rounding deepens with the dative -um: barna-börn-unum.
The takeaway is not to memorise every compound, but to see that the umlaut is local: each syllable rounds or not based on the u in its own following syllable, so a long word can mix a and ö.
A glimpse beyond nouns
Because it is a sound rule, the u-umlaut also shows up in verbs and adjectives wherever a stem a meets a following u. The clearest case is the 1st-person-plural verb ending -um:
Við tölum saman á hverjum degi.
We talk together every day. The verb 'tala' (to talk) → 'við tölum' — the 1pl ending -um rounds the stem 'a' to 'ö'.
Við förum á fætur klukkan sjö á morgnana.
We get up at seven in the mornings. 'fara' → 'við förum' — same -um trigger, same rounding.
So the single mental rule — a + following u → ö — pays off three times over: in the noun dative plural (dögum), in feminine/neuter plurals (sögur, lönd), and in the við-form of verbs (tölum, förum). It is the same rule each time.
Common Mistakes
❌ Við gistum á tveimur stöðum í þremur dagum.
Incorrect — 'dagum' fails to round; the dative-plural -um demands ö.
✅ Við gistum á tveimur stöðum í þremur dögum.
We stayed in two places over three days. 'dögum' — stem a rounds before -um.
❌ Ég hef búið í mörgum landum.
Incorrect — 'landum' must round; the neuter a-stem rounds in the dative plural.
✅ Ég hef búið í mörgum löndum.
I've lived in many countries. 'löndum' — a → ö before -um.
❌ Hún kann margar sagur.
Incorrect — the feminine a-stem rounds in the nominative/accusative plural too, not only the dative.
✅ Hún kann margar sögur.
She knows many stories. 'sögur' rounds because the plural ending -ur contains u.
❌ Rounding where there is no a: 'húsöm' for the dative plural of hús.
Incorrect — the stem is 'ú', so nothing rounds; the ending is simply -um.
✅ í gömlu húsum
In old houses. 'húsum' stays unrounded — no stem a to trigger the umlaut.
❌ börnanna written as 'börnunna' or the genitive plural rounded.
Incorrect — the genitive plural 'barnanna' has no u in its ending, so the stem stays a.
✅ leikföng barnanna, en gjafir handa börnunum
The children's toys, but gifts for the children. Genitive plural 'barnanna' keeps a; dative plural 'börnunum' rounds.
Key Takeaways
- The u-umlaut: a stem a rounds to ö when a u follows in the next syllable.
- The most reliable trigger is the dative-plural -um: dagur → dögum, land → löndum, hjá börnum.
- a-stem feminines and neuters also round in the bare/nominative plural: saga → sögur, land → lönd.
- No stem a means no rounding — hestum, húsum, fiskum stay put.
- The genitive plural usually ends in -a (no u), so it does not round: landa, barnanna.
- The same rule fires beyond nouns — the verb við-ending -um rounds too: tala → tölum, fara → förum.
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- U-Umlaut as a Sound Alternation (a → ö)A2 — When a u appears (or once appeared) in the next syllable, a stem 'a' is rounded to 'ö' — barn → börn, dagur → dögum, kalla → köllum. This is the living u-umlaut (u-hljóðvarp), an automatic, predictable rounding that explains why so many Icelandic paradigms 'change their vowel'.
- Neuter Nouns: The Core Pattern (borð, land)A2 — The strong neuter declension — the most uniform gender in Icelandic, where nominative and accusative are always identical, the plural adds no ending at all, and number is often carried only by the article, adjective or verb.