Single n vs Double nn in Endings

Here is a spelling decision your ear can sometimes help with and sometimes can't, and where guessing is costly because the choice carries grammatical meaning. Single -n and double -nn are not interchangeable variants like English -ise/-ize; in many endings they signal different cases or genders, so writing one for the other isn't a typo — it can change what the word means. The headline fact, and the one that fixes the most errors: the masculine singular definite article is -inn with double n (hesturinn "the horse"), while the neuter is -ið (húsið "the house"). Get the article paradigm into your hands and the n/nn question largely answers itself. (This page is about the spelling choice. The pronunciation of nn — when it's a long [nː] and when it's the [tn] of steinn — lives on pronunciation/ll-rl-nn-rn; here we only decide which letters to write.)

The masculine definite article is -inn (double n)

The most frequent place the double nn appears is the suffixed definite article on a masculine noun in the nominative singular: -inn. Hestur "a horse" → hesturinn "the horse"; bíll "a car" → bíllinn "the car"; strákur "a boy" → strákurinn "the boy." That article is always -inn with two n's. The single most common spelling error in this whole area is dropping one and writing *hesturin. Burn it in: nominative masculine definite = -inn.

That last example is the whole lesson in miniature. Strákurinn (subject, nominative) has the double-n article -inn; skólanum (after í, dative) has the single-n article -num. Same noun-plus-article machinery, two different cases, two different n-spellings. The double n is the badge of the nominative masculine form.

The neuter is -ið, not -inn

A frequent confusion: learners over-apply -inn to neuter nouns. But the neuter definite article is -ið (with ð, not nn): hús "a house" → húsið "the house"; barn "a child" → barnið "the child." There is no n at all in the neuter nominative/accusative article. So the gender of the noun decides the letter: masculine -inn, neuter -ið. Mixing them up writes the wrong gender onto the noun.

Notice hæðinni "on the hill" in the first example: that's the feminine dative article -inni, which does have double n. So the feminine has its own double-n slots too — the point is never to default to one spelling, but to read the gender and case off the ending.

Oblique-case articles take single -n

When the noun (and its article) leave the nominative, the article's n often halves. The masculine dative is -num (single n): hestinum "to/from the horse." The genitive plural is -nna (double n again — hestanna "of the horses"), but the singular obliques and several others run on a single n. The takeaway is mechanical: you cannot carry the double n of -inn into the oblique cases; each case has its own fixed article shape.

So within a single noun's life you meet both: nominative/accusative hesturinn/hestinn with nn, dative hestinum with single n. The double n is not a property of the word hestur — it's a property of the nominative (and accusative) definite slot. Learn the four-case article row above and you stop guessing.

-nn in minn, þinn, sinn and einn

Outside the article, double nn shows up in a small, very high-frequency set of words you should simply know. The possessives minn "my," þinn "your," sinn "his/her/their own" end in -nn in the masculine nominative — minn bíll, bíllinn þinn. The number/determiner einn "one, a" likewise has -nn: einn maður "one man." These pattern like the article: the double n is the masculine nominative form, and it reduces in other cases — minn but mínum (dative), einn but einum (dative), eina (feminine accusative).

The feminine and neuter of these words change shape and lose the double n in telling places — mín (f. nom., single n), eina (f. acc.), eitt (n.). So again the n/nn tracks gender and case, not the lexical identity of the word.

-nn after a stressed long vowel: seinn

There is also a phonological pocket: certain adjectives and words ending in a stressed long vowel take -nn in the masculine nominative — seinn "late," hreinn "clean," grænn "green," steinn "stone," brúnn "brown." The double n here pairs with the long vowel before it. The neuter of these often shows the contrast sharply: seinn (m.) but seint (n.), hreinn (m.) but hreint (n.) — the nn belongs to the masculine, and the neuter replaces it with -t.

The pair seinn (masculine) vs seint (neuter/adverb) is a perfect illustration: the nn is grammatically meaningful, marking the masculine adjective, and swapping to the neuter swaps -nn for -t.

Why it's meaningful, not free

Pull the thread and the principle is consistent: in Icelandic grammatical endings, -n vs -nn encodes information — usually case and gender. Hesturinn (nom.) vs hestinum (dat.); seinn (m.) vs seint (n.); minn (m. nom.) vs mín (f. nom.). English has nothing comparable, because English barely inflects and its rare double letters (running, winner) are about syllable structure, not grammar. So the instinct an English speaker needs is the reverse of "spell it however looks right": decide the case and gender first, and the n/nn falls out of the paradigm. The defence against both errors — dropping an n in -inn and sprinkling extra n's elsewhere — is knowing the article and possessive paradigms cold. (For more trap-spelling practice, see spelling/practice-traps.)

Common Mistakes

The nominative masculine article is -inn. There is no single-n version of it; *hesturin is simply wrong.

Don't paste the masculine -inn onto a neuter noun. Neuter is -ið, which has no n at all.

Over-doubling in the oblique cases is the mirror error. The dative is -num, single n; the double n is exclusive to the nominative/accusative.

minn (m.) vs mín (f.) differ by exactly the n/nn — so the wrong spelling assigns the wrong gender. Bíll is masculine, so it's minn.

Key Takeaways

• -n vs -nn is grammatically meaningful in Icelandic endings — it usually encodes case and gender, not free variation.
• The masculine nominative/accusative definite article is -inn (double n): hesturinn, hestinn. Dropping an n (*hesturin) is the top error.
• The neuter article is -ið (no n): húsið, barnið — don't use -inn on neuters.
• Oblique cases take single -n: dative -num (hestinum), so you can't carry the nominative's double n through the paradigm.
• -nn also marks the masculine nominative of minn / þinn / sinn, of einn, and of words after a stressed long vowel (seinn, hreinn, grænn) — each reducing in other genders/cases (mín, eina, seint).
• The fix for both under- and over-doubling: learn the article and possessive paradigms, then read the n/nn off the case and gender.