Dekkin eru orðin gömul, svo hún pantar ný áður en veturinn byrjar.

Because dekk is the noun tire, and dekkin is its definite plural form.

• dekk = tire / tires
• dekkin = the tires

In Icelandic, the definite article is usually attached to the end of the noun instead of being a separate word like the in English.

So:

• dekk = tires
• dekkin = the tires

Here dekkin is the subject of the sentence.

Because eru orðin gömul means have become old or have gotten old, not just are old.

Compare:

• Dekkin eru gömul = The tires are old.
• Dekkin eru orðin gömul = The tires have become old / have gotten old.

So orðin adds the idea of a change of state. It tells you that the tires were not always old, but now they have reached that state.

Orðin is the past participle of verða, which means become.

In this pattern:

• vera + orðinn + adjective

the meaning is often have become.

So:

• eru orðin gömul literally resembles are become old
• but in natural English it means have become old

This is a very common Icelandic way to express a resulting change.

They agree with dekkin, which is neuter plural.

In Icelandic, adjectives and participles often agree with the noun they describe in:

• gender
• number
• sometimes case

Here:

• dekkin = neuter plural
• orðin = neuter plural
• gömul = neuter plural

So the grammar is matching all the way through.

Yes. Gömul is a form of the adjective gamall, which means old.

This adjective is irregular, so its forms can look quite different. Some common forms are:

• gamall = old (masculine singular)
• gömul = old (feminine singular / neuter plural)
• gamalt = old (neuter singular)

So in this sentence, gömul is used because dekkin is neuter plural.

Here svo means so, therefore, or as a result.

It connects the two parts of the sentence:

• Dekkin eru orðin gömul = The tires have become old
• svo hún pantar ný = so she orders new ones

So it shows cause and result.

Because this svo is acting like a coordinating conjunction meaning so.

After that kind of svo, normal main-clause order is used:

• hún pantar = she orders

So:

• ..., svo hún pantar ný ... = ..., so she orders new ones ...

This is different from cases where an adverb or fronted element triggers normal Icelandic verb-second inversion.

In Icelandic, the adjective can stand on its own when the noun is obvious from context.

So:

• hún pantar ný = she orders new ones

The noun dekk is understood, so it is left out.

You could also say:

• hún pantar ný dekk = she orders new tires

But in this sentence, Icelandic simply leaves dekk unstated because it is already clear.

Because the missing noun is understood to be dekk, which is neuter plural, and it is the direct object of pantar.

So the adjective has to match that understood noun.

In other words:

• understood noun: dekk
• gender/number: neuter plural
• adjective form: ný

This is why you do not get forms like nýr, nýja, or nýtt here.

Áður en means before and introduces a subordinate clause.

So:

• áður en veturinn byrjar = before winter begins

A useful way to remember it is:

• áður = earlier / before
• en = than / that / introducing the clause here

Together they function as before.

Because veturinn means the winter, and here Icelandic is referring to the specific upcoming winter.

English often says simply before winter begins, without the, but Icelandic commonly uses the definite form in this kind of context:

• veturinn = the winter

So the sense is:

• before the winter begins
• or more naturally in English, before winter begins

Because Icelandic often uses the present tense for future events, especially when the future event is expected or scheduled.

So:

• veturinn byrjar literally = the winter begins
• but in context it means the winter will begin

This is very similar to English in sentences like:

• I’ll call you before the movie starts.

English also uses a present form in that kind of time clause, and Icelandic does the same here.