Forming the Genitive Across Classes

The genitive is the most variable case in Icelandic — it is the one place where the ending depends most sharply on which declension class a noun belongs to. Where the accusative and dative often share forms or stay close, the genitive singular forks into at least four different endings (-s, -a, -ar, -u), and choosing the wrong one is the single most common case error English speakers make, because English has exactly one genitive marker (-'s) and the instinct is to map it onto Icelandic's -s. That instinct works for masculines and neuters and fails everywhere else. This page is a formation reference: it lays out, class by class, what the genitive singular and plural endings are, so you can look up any noun's genitive in one place. (For what the genitive is used for — possession, the partitive, governance by til / án / vegna — see the genitive uses page.)

The one rule that always holds: genitive plural -a

Before the forks, the consolation. The genitive plural is almost always -a, across every class: hesta (horses'), borga (cities'), barna (children's), tíma (hours'), gesta (guests'). Whatever a noun's genitive singular does — and it does many different things — its genitive plural is overwhelmingly -a. There is a small but important variant, -na, used by weak nouns and a handful of feminines (kvenna "women's," tungna "tongues'"), which we cover below. But the headline is reassuring: even when the singular genitive of a hard noun stumps you, the plural is highly regular. Lock in -a as your default genitive plural and you are right far more often than not.

The master table

Here is the genitive of one model noun from each class, in both numbers. Read it as a lookup chart — these are the patterns every other noun in the class follows:

The pattern beneath the table: masculines and neuters take -s in the genitive singular (hests, gests, borðs); strong feminines take -ar (borgar); weak nouns take the weak vowels — weak masculine -a (tíma), weak feminine -u (konu). The genitive plural is -a everywhere except the weak-feminine -na type. So the genitive singular is where you must know the class; the genitive plural is nearly free.

Strong masculine and neuter: -s

Both strong-masculine subclasses — the -ar type and the -ir type — and the strong neuters take -s in the genitive singular and -a in the genitive plural. This is the class that matches English's -'s, which is why it feels easy:

Two cautions inside this easy class. First, some masculines take -ar, not -s, in the genitive singular even though they look like the -s type — staður → staðar, fjörður → fjarðar, vetur → vetrar. You learn the genitive singular with the word; the plural type does not predict it. Second, many neuters and masculines whose stem ends in -s or a consonant cluster simply add -s with no fuss, but a few syncopate (aldur → aldurs, hamar → hamars but himinn → himins) — again a per-word fact.

Strong feminine: -ar

The strong feminines take -ar in the genitive singular — borg → borgar, mynd → myndar, bók → bókar — and the regular -a in the plural. This -ar is the ending English speakers most often get wrong, because the -s reflex from English overrides it. There is no -s on a feminine genitive; it is -ar:

Weak nouns: -a (masculine), -u (feminine)

Weak nouns — those whose nominative singular ends in a vowel — take weak vowel endings in the genitive singular, not consonantal -s. A weak masculine like tími ("hour, time") or kennari ("teacher") ends its genitive singular in -a (tíma, kennara); a weak feminine like kona ("woman") or saga ("story") ends it in -u (konu, sögu). The genitive plural is -a for weak masculines (tíma) but the -na variant for weak feminines (kvenna, sagna):

Note that kona is doubly irregular in the plural: it not only takes the -na ending but changes its stem to kvenn- (gen.pl kvenna, like the whole plural konur / konur / konum but genitive kvenna). The ordinary weak feminines are tamer: saga → sagna, tunga → tungna — the -na still appears, and a doubled -n- is the signature of this genitive plural.

i-umlaut in the genitive singular of monosyllabic feminines

There is one place the genitive does not just add an ending but changes the stem vowel. The high-frequency monosyllabic feminines — hönd, mörk, nótt — take the strong-feminine -ar genitive, but on the bare, un-umlauted stem vowel, which surfaces as something different from the nominative. hönd ("hand") has nominative hönd (u-umlauted ö) but genitive handar — the underlying a shows through because the genitive ending never triggered u-umlaut. Likewise mörk → markar, nótt → nætur (this one fronts instead). So the genitive singular of these nouns is where the real stem vowel is visible:

Proper names take the genitive too

Place names and personal names inflect like any noun, and they appear in the genitive constantly — after til ("to"), in possession, on book spines and maps. The ending follows the name's own class:

Note Reykjavík → Reykjavíkur especially: it does not take -s. The second element -vík ("bay") is a feminine noun, and its genitive surfaces as -ur on this stem, so the whole name's genitive is Reykjavíkur. This is exactly the trap the English -'s instinct sets — and exactly why proper-name genitives deserve their own proper nouns page.

Why "just add -s" fails

English collapses the genitive into a single clitic: the horse's, the city's, the woman's, the table's — -'s everywhere, blind to the noun. Icelandic refuses that uniformity. The genitive singular ending is a function of declension class, and there are four live options — -s (masculine/neuter), -ar (strong feminine), -a (weak masculine), -u (weak feminine). Defaulting to -s gives you the right answer only for masculines and neuters; it produces *borgs for borgar, *konus for konu, *tímas for tíma. The discipline, then, is to learn the genitive singular as part of the word, alongside its gender and plural — and to lean on the one reliable shortcut, genitive plural -a, whenever you are dealing with more than one of something.

Common Mistakes

Key Takeaways

• The genitive plural is the safe part: almost always -a (hesta, borga, barna), with a -na variant for weak feminines and a few others (kvenna, sagna).
• The genitive singular forks by class: masculine/neuter -s (hests, borðs); strong feminine -ar (borgar); weak masculine -a (tíma); weak feminine -u (konu).
• English "just add -s" fails for feminines (borgar, not borgs) and weak nouns (konu, tíma) — learn the genitive singular with the word.
• Monosyllabic feminines show their bare stem vowel in the genitive: hönd → handar, mörk → markar (the ö un-rounds to a).
• Proper names inflect: Ísland → Íslands, Jón → Jóns, but Reykjavík → Reykjavíkur (feminine -ur, not -s).