Strong Masculine: r-stems and Irregulars (fótur, maður, fjörður)

Most strong masculines fall into the two big plural camps — the -ar type (hestur → hestar) and the -ir type (gestur → gestir). This page is about the masculines that escape both: a small set of irregulars whose plurals you simply have to know as whole paradigms. They matter out of all proportion to their number because they are some of the most common nouns in the language — maður ("man, person"), fótur ("foot, leg"), fjörður ("fjord"), vetur ("winter"), fingur ("finger"). You cannot build these from a rule; the rule-based subclasses are a separate page. Here you memorise. The reward is that once these handful are solid, the rest of the masculine system is the predictable -ar/-ir machinery.

The i-umlaut plurals: fótur → fætur

A small group of masculines build their plural not by adding -ir or -ar but by fronting the stem vowel — the same i-umlaut that turns English foot → feet and man → men. In fact fótur → fætur is the Icelandic cousin of foot → feet: both come from the same prehistoric process. The vowel ó fronts to æ in the plural, and the plural ending is -ur (nominative/accusative):

Read the distribution carefully, because it is exactly like the feminine hönd. The æ appears where a historical i-ending sat: the nominative and accusative plural (fætur) and the dative singular (fæti). Everywhere else the ó survives — fótur, fót, fótar in the singular and fótum, fóta in the plural. So the umlaut is not "the plural fronts"; it is "the i-coloured cells front." That is why the dative plural is fótum (with the original ó), not *fætum — the -um ending never triggered i-umlaut.

The no-change plurals: vetur and fingur

The opposite trap is just as common: a couple of high-frequency masculines do not change at all in the nominative/accusative plural. Vetur ("winter") and fingur ("finger") look identical in the singular and the plural nominative — you tell number only from the article or the verb agreement. They are r-stems whose nominative singular already ends in -ur as part of the stem, not as an ending, so there is nothing to add:

So "three winters" is þrír vetur — the noun does not move; only the numeral and the article (veturnir "the winters") show plurality. The only place these change shape is the dative, where the -u- of the stem drops out before the ending: vetri / vetrum, fingri / fingrum (this is just the ordinary syncope you see in aldur → aldri). Note the two genitive singulars differ — vetrar (-ar) but fingurs (-s) — which, as always with masculines, you learn with the word.

maður: the most important irregular in the language

maður ("man, person, human being, one") deserves its own heading because it is, on most counts, the single most frequent noun in Icelandic — it does the work of English "man," "person," "people," and the impersonal "one/you" all at once. And its plural is suppletive: it comes from a completely different stem. There is no derivation, no umlaut rule that gets you there. You learn maður → menn the way you learn go → went in English — as raw fact, on day one.

Three things to lock in. First, the singular stem is mann- everywhere except the bare nominative maður — so accusative mann, dative manni, genitive manns. Second, the plural switches to the suppletive menn- stem in the nominative and accusative (menn) and to mann- in the genitive (manna). Third — and this catches everyone — the dative plural is mönnum, with u-umlaut: the a of mann- rounds to ö before the -um. So the four plural forms are menn / menn / mönnum / manna, and only the genitive manna keeps the a unrounded.

Watch out, too, for the impersonal maður — "you / one / people in general," as in maður veit aldrei ("you never know"). It uses the same forms and is everywhere in speech; treat it as a pronoun-like use of the same word, not a different noun.

The r-stems: fjörður and the ö → a → i run

The classic Icelandic r-stems are masculines whose nominative singular ends in -ur that is really a leftover -r on a vowel-final stem, and whose plural is the -ir type but with a stem-vowel change. fjörður ("fjord, inlet") is the model, and it is everywhere in place names — Ísafjörður, Hafnarfjörður, Seyðisfjörður — so its declension is not optional knowledge. It cycles through three stem vowels: ö in the bare singular, a in the genitive singular, and i in the plural:

The three vowels are not chaos — each is the lawful output of its ending. The underlying stem vowel is a, and you see it bare in the genitive (fjarðar, fjarða), whose endings never umlauted. A historical u in the ending rounds it to ö: the nominative singular fjörður (a vanished -u-) and the dative plural fjörðum (the visible -um). A historical i in the ending fronts it to i: the whole plural firðir / firði and the dative singular firði. So fjörður / fjörð / fjarðar / firðir / firði / fjörðum / fjarða is one a-stem run through u-umlaut and i-umlaut in the right cells — the same machine as the feminine hönd, just with the genitive showing the bare vowel.

This is why the place-name genitives look the way they do: Ísafjörður → Ísafjarðar ("of Ísafjörður"), Seyðisfjörður → Seyðisfjarðar. When you say "I'm going to Hafnarfjörður," the til-phrase needs the genitive Hafnarfjarðar, not the nominative.

Why English speakers must memorise these

English froze its vowel-change plurals into a tiny closed museum — foot/feet, man/men, tooth/teeth, mouse/mice — and then derives everything else with -s. Two things surprise the English speaker here. First, Icelandic kept this machinery productive and frequent: the very words that are irregular in English (foot, man) are irregular in Icelandic too (fótur → fætur, maður → menn), but so are many more (fjörður → firðir, vetur unchanged). Second, the change is distributed by case, not just singular-vs-plural — fótur fronts in the dative singular fæti as well as the plural, and fjörður shows a different vowel in three different cells. So you cannot learn "the plural" and stop; you learn the whole paradigm as a unit. The consolation is that these are a closed, countable set of common words, and they reward memorisation immediately because you meet them every single day.

Common Mistakes

Key Takeaways

• These irregulars cannot be derived — memorise the whole paradigm, including the dative plurals, because the high-frequency words live here.
• fótur → fætur fronts ó to æ in the i-cells (dat.sg fæti, nom./acc.pl fætur); the dative plural stays fótum with ó.
• vetur and fingur do not change in the nominative/acc. plural (þrír vetur "three winters"); they only shift in the dative (vetri/vetrum, fingri/fingrum).
• maður → menn is suppletive — learn it from day one: sg maður / mann / manni / manns, pl menn / menn / mönnum / manna (dative plural mönnum with u-umlaut).
• fjörður is the model r-stem, running ö → a → i: fjörður / fjörð / fjarðar / firðir / firði / fjörðum / fjarða. Place-name genitives follow it (Hvalfjarðar).