Ég skafa rúðuna á hverjum morgni þegar það er kalt.

Because rúðuna is the definite direct object of skafa.

• rúða = the dictionary form
• rúðu = accusative singular indefinite
• rúðuna = accusative singular definite

The verb skafa takes a direct object in the accusative, so the noun changes form. Icelandic usually puts the on the end of the noun, which is why you see -na instead of a separate word.

This is an idiomatic time expression meaning every morning.

Literally, it is something like on each morning:

• á = on
• hverjum = each / every, in the dative singular masculine
• morgni = morning, in the dative singular

So the whole phrase uses á + dative.

Morgni is the dative singular of morgunn.

The noun changes like this:

• nominative: morgunn
• accusative: morgun
• dative: morgni
• genitive: morguns

Since á takes the dative here, you get morgni. The word hverjum also has to match that case, so it becomes hverjum.

With á, Icelandic often uses:

• accusative for motion onto something
• dative for location or a fixed setting

In time expressions like á hverjum morgni, the idea is not movement but a recurring time point, so dative is used.

It is a dummy subject, just like English it in it is cold.

It does not refer to a specific thing. Icelandic often uses það in weather, temperature, and general condition expressions:

• það er kalt
• það er heitt
• það rignir

So here það is just the grammatical subject needed for the clause.

Because the adjective is being used in an impersonal expression, so Icelandic normally uses the neuter singular form.

Compare:

• bíllinn er kaldur = masculine
• súpan er köld = feminine
• það er kalt = neuter singular, used impersonally

So kalt is the normal form in expressions like this.

For many Icelandic verbs, the 1st person singular present has the same form as the infinitive without að.

• infinitive: að skafa
• present: ég skafa

Other persons show endings more clearly:

• þú skafar
• hann / hún / það skafar
• við sköfum

So ég skafa is completely normal.

Because the sentence describes a habitual action. Icelandic uses the present tense for things you do regularly, just like English does:

• Ég les á kvöldin
• Hún hleypur á hverjum degi
• Ég skafa rúðuna á hverjum morgni...

There is no special separate habitual form needed.

Because þegar introduces a subordinate clause.

In a main clause, Icelandic usually follows the verb-second rule. But after a subordinating word like þegar, the order is normally:

subordinator + subject + verb

So:

• þegar það er kalt

not

• þegar er það kalt

Yes. Þegar can also mean already in other contexts.

For example:

• Ég er þegar búinn = I am already finished

But in this sentence, þegar is followed by a full clause, so it is clearly the conjunction when.

Yes. Icelandic is flexible, and this is also natural:

Á hverjum morgni skafa ég rúðuna þegar það er kalt.

When a time phrase comes first in a main clause, the finite verb still stays in second position, so skafa comes before ég.

That is a very common Icelandic word-order pattern.

Normally, no. Icelandic usually keeps the subject pronoun, especially in ordinary statements.

So:

• Ég skafa rúðuna... = normal

Leaving out ég would usually sound incomplete unless the subject is understood from a very specific context, such as a diary style, instructions, or coordinated clauses.