Learn Hardcore Icelandic: Ég klæðist þykku jakkanum ef veðrið er kalt. - I wear the thick jacket if the weather is cold.

klæðast is an intransitive “middle-voice” verb that means “to don/wear” and takes its object in the dative. You say ég klæðist jakkanum (I wear the jacket). By contrast, klæða sig í is a reflexive, transitive construction: ég klæði mig í jakka literally “I dress myself in a jacket,” and the jacket then takes the accusative case.

What case is þykku jakkanum, and why is it in that case?

It’s in the dative singular definite. The verb klæðast governs the dative for the garment (þykka jakka → þykka jakkanum when definite).

Why does the adjective þykkur become þykku here?

Because after a definite noun the adjective weakly declines. In masculine dative singular, the weak form of þykkur is þykku.

Why does jakki end up as jakkanum rather than jakkinum or jakkinum?

jakki is a masculine noun. In the dative singular definite, the ending is -anum, but because the stem ends in -k, the full form is jakkanum (jakki → jakk- + anum = jakkanum).

Why is veðrið used here instead of just veður?

veðrið is the definite form of veður (“the weather”), marked by the suffix -ið in neuter nominative. We need “the weather” in a specific sense.

Why is the adjective kalt not inflected like kalda or kaldur?

Here kalt is a predicate adjective following a neuter nominative subject (veðrið). Predicate adjectives agree in gender and number with the subject: neuter nominative singular of kaldur is kalt.

What is the word-order rule for the subordinate clause introduced by ef?

In Icelandic, ef is a subordinating conjunction that does not count toward the V2 (verb-second) rule. You still put the finite verb in second position, so you get ef veðrið er kalt (conjunction → subject → verb → predicate).

How do you pronounce the double kk in jakkanum?

Double kk indicates a long (geminate) voiceless [kː]. It’s held slightly longer than a single k and isn’t aspirated like English /k/.

Could you say ef veðrið væri kalt instead? What changes?

Yes. væri is the past-tense (subjunctive) form of vera, so ef veðrið væri kalt gives your clause a more hypothetical or counterfactual tone (“if the weather were cold”) rather than a simple present‐time condition.

Could you also say Ég klæði mig í þykka jakkann ef veðrið er kalt? What’s the difference?

Absolutely. That uses klæða sig í plus accusative (þykka jakkann) and the reflexive pronoun mig. Meaning is virtually the same; choice comes down to style and which construction you prefer.