The Weak (Definite) Declension | Icelandic Grammar

After the effort of the strong declension, the weak declension is a relief. Where the strong set has a different ending for nearly every gender-case-number combination, the weak set has just three shapes in the singular — -i and -a (with -u in the feminine oblique) — and a single shape for the whole plural: -u. The catch is not the forms; it's knowing when to use them. The weak declension is the definite declension: you use it when the adjective sits inside a noun phrase that is already marked as definite — by the suffixed article, by a demonstrative (þessi "this," sá "that"), or by a possessive (minn "my"). This page gives you the complete weak paradigm for gamall ("old") and drills the one idea English speakers keep missing: that the definite phrase is double-marked — the weak adjective and the noun's article both say "definite" at once.

The rule: definiteness picks the declension

The strong-vs-weak choice has nothing to do with the adjective's meaning and everything to do with the definiteness of the whole noun phrase:

Indefinite ("an old man") → strong: gamall maður.
Definite ("the old man") → weak: gamli maðurinn.

A phrase counts as definite — and so triggers the weak adjective — in three cases:

The noun carries the suffixed article: gamli maðurinn ("the old man").
The phrase opens with a demonstrative: þessi gamli maður ("this old man"), sá gamli maður ("that old man").
The phrase opens with a possessive: gamli vinur minn ("my old friend").

In all three, the adjective goes weak. The trigger is somewhere in the phrase; the adjective just reads it.

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Don't look at the adjective to decide strong or weak — look at the phrase. Is there an article, a þessi/sá, or a minn/þinn? Then the adjective is weak. No such marker? Strong. The adjective is following orders, not giving them.

The full weak paradigm: gamall

Here is gamall in its weak forms. Notice immediately how few distinct endings there are — the whole plural is one form, and the singular has only three:

Case	Masculine sg	Feminine sg	Neuter sg	Plural (all genders)
Nefnifall (nom.)	gamli	gamla	gamla	gömlu
Þolfall (acc.)	gamla	gömlu	gamla
Þágufall (dat.)	gamla	gömlu	gamla
Eignarfall (gen.)	gamla	gömlu	gamla

Read the grid and the economy is striking. The masculine nominative singular is the only -i form: gamli. Everywhere else in the masculine and neuter singular it's -a: gamla. The feminine singular is gamla in the nominative and then gömlu in the three oblique cases (accusative, dative, genitive). And the entire plural — all three genders, all four cases — is gömlu. That's it. Four distinct strings — gamli, gamla, gömlu — cover the whole declension.

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Memorise the weak declension as a slogan: "-i only in the masculine nominative singular; -a almost everywhere else; -u in the feminine oblique and the whole plural." For gamall: gamli (one place), gamla (most places), gömlu (feminine oblique + plural).

The u-umlaut shows up wherever the ending is -u: gamall rounds its stem a → ö, giving gömlu in the feminine oblique singular and right across the plural. Many adjectives without an a in the stem never show this (the weak forms of fallegur are just fallegi / fallega / fallegu, no rounding) — but for a-stems like gamall, kaldur, svartur, the -u endings round.

The double-marking: the adjective does NOT replace the article

Here is the point most references skate over and most English speakers get wrong. When you make a definite phrase with an adjective, you mark definiteness twice: once on the adjective (it goes weak) and once on the noun (it keeps its suffixed article). The weak adjective is not a substitute for the article — both appear.

Gender	Indefinite (strong, no article)	Definite (weak + article)
Masculine	gamall maður	gamli maðurinn
Feminine	gömul kona	gamla konan
Neuter	gamalt hús	gamla húsið

Look at gamli maðurinn: the adjective is weak (gamli) and the noun keeps its article (-inn). Two definiteness signals, one phrase. English does this with a single "the" at the front; Icelandic spreads it across both words. Beginners systematically under-produce this — they say gamli maður (weak adjective, but they forgot the article) — which is ungrammatical. If the adjective is weak, the noun must wear its article.

Gamli maðurinn á horninu þekkir alla í hverfinu.

The old man on the corner knows everyone in the neighbourhood. Weak 'gamli' AND article 'maðurinn' — both definite.

Gamla konan í næsta húsi bakar bestu kleinur í bænum.

The old woman in the next house bakes the best doughnuts in town. Weak feminine 'gamla' + article 'konan'.

Gamla húsið er friðað og má ekki rífa.

The old house is protected and can't be demolished. Weak neuter 'gamla' + article 'húsið'.

Gömlu mennirnir á kaffihúsinu spila brids á hverjum degi.

The old men at the café play bridge every day. Weak plural 'gömlu' (note the u-umlaut) + article 'mennirnir'.

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If the adjective is weak, the noun keeps its article: gamli maðurinn, not *gamli maður. Picture the definite phrase as a sandwich — weak adjective on one side, suffixed article on the other. Drop either slice and it falls apart.

Demonstratives and possessives trigger it too — sometimes without an article

The suffixed article isn't the only thing that makes a phrase definite. A demonstrative or a possessive does the job, and then the adjective goes weak even if the noun has no article suffix:

Þessi gamli bíll fer aldrei í gang á veturna.

This old car never starts in the winter. After 'þessi' the phrase is definite → weak 'gamli', and 'bíll' has no article.

Sá gamli draumur rættist loksins.

That old dream finally came true. Demonstrative 'sá' → weak 'gamli'.

Hann hitti gamla vini sína á ættarmótinu.

He met his old friends at the family reunion. Possessive 'sína' makes the phrase definite → weak plural 'gamla' (acc. masc. pl.).

These show the rule at its purest: there's no article on bíll, draumur, or vini, yet the adjective is weak — because þessi, sá, and sína already supplied the definiteness.

A few oblique cases in context

The weak forms in their case work, so the -a and -u aren't just nominative shapes:

Ég talaði við gamla manninn í gær.

I spoke to the old man yesterday. Accusative masculine weak 'gamla' (+ article 'manninn').

Hún býr hjá gömlu konunni í kjallaranum.

She lives with the old woman in the basement. Dative feminine weak 'gömlu' (the -u form) + article 'konunni'.

Common Mistakes

❌ Gamall maðurinn gengur hægt.

Incorrect — the noun is definite (article -inn), so the adjective must be WEAK, not the strong 'gamall'.

✅ Gamli maðurinn gengur hægt.

The old man walks slowly. Definite phrase → weak 'gamli'.

❌ Gamli maður gengur hægt.

Incorrect — you used the weak adjective but dropped the article. A weak adjective REQUIRES the article on the noun.

✅ Gamli maðurinn gengur hægt.

The old man walks slowly. Weak adjective + article, both present.

❌ Þessi gamall bíll er bilaður.

Incorrect — after 'þessi' the phrase is definite, so the adjective goes weak: gamli.

✅ Þessi gamli bíll er bilaður.

This old car is broken. Demonstrative → weak 'gamli'.

❌ Hún býr hjá gamla konunni.

Incorrect — the dative feminine weak of 'gamall' is 'gömlu' (with u-umlaut), not 'gamla'.

✅ Hún býr hjá gömlu konunni.

She lives with the old woman. Feminine oblique weak 'gömlu'.

❌ Gamli mennirnir spila brids.

Incorrect — the plural weak is 'gömlu' (one form for the whole plural), not the masculine-singular 'gamli'.

✅ Gömlu mennirnir spila brids.

The old men play bridge. Weak plural 'gömlu'.

Key Takeaways

The weak declension is the definite declension — used when the phrase is definite via the article, a demonstrative (þessi/sá), or a possessive (minn).
It has almost no forms: -i only in the masculine nominative singular (gamli), -a almost everywhere else, -u in the feminine oblique and the entire plural (gömlu).
A definite phrase is double-marked: the adjective goes weak and the noun keeps its article — gamli maðurinn. Never drop the article when the adjective is weak.
Demonstratives and possessives trigger weak adjectives even with no article on the noun: þessi gamli bíll.
For a-stems, the -u endings show u-umlaut: gamall → gömlu.

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