Kinship Nouns: -ir Stems (faðir, móðir, bróðir)

Five of the most common nouns in the language belong to a tiny declension class shared with almost no other words: the kinship nouns in -ir — faðir ("father"), móðir ("mother"), bróðir ("brother"), dóttir ("daughter"), and systir ("sister"). They are irregular in two different places at once: the singular has an oblique stem in -ur/-ður (so "father" in object position is föður, not *faðir), and the plural changes its stem vowel by i-umlaut (feður, mæður, bræður, dætur). Because these are words you need from your first week, the efficient move is to learn the whole five-member class as a block — get the pattern once and you unlock all of close-family vocabulary together. (The wider topic of irregular plurals across all classes has its own page; here we focus on this single tight family.)

The shape of the class

All five share a profile. The nominative singular ends in -ir (faðir, móðir, bróðir, dóttir, systir) — and that -ir is not a plural; it is the citation form, the dictionary entry. The oblique singular (accusative, dative, genitive) drops the -ir and ends in -ur or -ður instead. And the plural fronts the stem vowel by i-umlaut for four of the five — only systir resists it.

Two things to read off this table immediately. First, the oblique singular is the same across accusative, dative, and genitive — föður covers all three for "father" (you say ég sá föður minn "I saw my father," hjá föður mínum "at my father's," hús föður míns "my father's house," all with föður). That is a gift: one form does three jobs. Second, the plural and the singular look genuinely different because of the umlaut — faðir but feður, móðir but mæður — so you cannot guess the plural from the citation form by rule; you learn it.

faðir: the full paradigm

Faðir is the one with the a → e fronting (the other umlauting members go ó → æ), so it is worth seeing whole. Note the ð in the oblique stem föð- — it is never word-initial, and here it sits comfortably between vowels:

So the singular is faðir / föður / föður / föður — the nominative faðir stands alone, and the other three cases all become föður. The plural is feður / feður / feðrum / feðra — the e of the umlaut runs through the whole plural, with the dative feðrum and genitive feðra syncopating the second vowel (feð-r-um, not *feðurum).

móðir: the full paradigm

Móðir runs the ó → æ fronting that the other three umlauting members share (bróðir, dóttir work identically), so its paradigm is the model for the -óðir/-óttir type:

The shape mirrors faðir exactly, only with ó/æ instead of a/e: singular móðir / móður / móður / móður, plural mæður / mæður / mæðrum / mæðra. (These nouns are grammatically feminine, which is why the article forms differ from faðir's — móðirin, móðurinni — but the noun's own endings follow the same kinship template.) Memorise móðir → mæður alongside faðir → feður and you have the engine for bróðir → bræður and dóttir → dætur for free.

bróðir, dóttir, systir: the rest of the family

Bróðir and dóttir follow móðir precisely — ó → æ in the plural — while systir is the outlier that takes the kinship endings but does not umlaut, so its plural systur is identical in vowel to the singular stem:

Watch systir carefully: its oblique singular systur and its nominative plural systur are spelled identically — you tell them apart only by context and agreement (systur mína "my sister," accusative singular, vs systur mínar "my sisters," accusative plural). The other four never have this collision, because their umlaut keeps singular and plural visibly distinct.

A genitive in action

Because the oblique singular does triple duty, the genitive of these nouns is just the same -ur/-ður form again. "My father's son" is sonur föður míns — föður is the genitive singular, and the possessive míns agrees with it in the genitive:

This is why the class is worth learning whole: once föður and móður are automatic, the genitive — the case English speakers most often fumble — costs you nothing extra, because it reuses the oblique form you already know.

Why these five cluster together

The five kinship nouns are a direct inheritance from Old Norse, where they formed a distinct r-stem declension; modern Icelandic preserves the class almost untouched. That history is why they behave alike and why no productive rule generates them — you cannot derive feður from faðir by a live process any more than English derives brethren from brother. The compensating fact is that they are a closed set of exactly five, all extremely frequent, all built on the same skeleton: nominative -ir, oblique -ur/-ður, umlaut plural (bar systir). Learn them as one paradigm with five fillings and you have covered the entire immediate family in a single sitting. Note, too, that more distant relatives do not join this class — frændi ("uncle / male relative") and frænka ("aunt / female relative") are ordinary weak nouns (frændi → frændur, frænka → frænkur), and amma ("grandmother") and afi ("grandfather") are plain weak nouns as well. The -ir template is reserved for these five.

Common Mistakes

Key Takeaways

• The five kinship nouns faðir, móðir, bróðir, dóttir, systir form one tight irregular class — learn them as a set.
• The nominative singular ends in -ir and is a citation form, NOT a plural; the oblique singular (acc./dat./gen.) is föður, móður, bróður, dóttur, systur — one form for all three non-nominative cases.
• The plural umlauts for four of the five: feður (a → e), mæður, bræður, dætur (ó → æ). Only systir → systur does not umlaut.
• systur is both the oblique singular of "sister" and the nominative plural "sisters" — disambiguate by agreement (mína vs mínar).
• The genitive reuses the oblique form (sonur föður míns), so mastering föður/móður hands you the genitive for free.
• More distant kin (frændi, frænka, amma, afi) are ordinary weak nouns, not members of this class.