Ef gallabuxurnar hefðu verið ódýrari, hefði hún líka keypt hettupeysuna.

This is a past unreal or counterfactual conditional.

It describes a situation that did not actually happen:

• the jeans were not cheaper
• as a result, she did not buy the hoodie either

So the sentence is talking about an imagined past situation and its imagined result.

Because Icelandic normally uses the past subjunctive in both parts of a past counterfactual sentence.

Here that gives:

• Ef gallabuxurnar hefðu verið ódýrari = if the jeans had been cheaper
• hefði hún líka keypt hettupeysuna = she would also have bought the hoodie

An English speaker may expect a separate verb for would have, but Icelandic very often uses hefði/hefðu + past participle in this kind of sentence.

They agree with different subjects:

• gallabuxurnar is plural, so you get hefðu
• hún is singular, so you get hefði

So the difference is just subject agreement in the past subjunctive of hafa.

Yes. Gallabuxur is a plural noun, just like English jeans or trousers.

So even when you mean one pair of jeans, Icelandic still treats it as plural:

• gallabuxur = jeans
• gallabuxurnar = the jeans

The ending -nar is the attached definite article, so the word literally means the jeans.

Ódýrari is the comparative form of ódýr, which means cheap.

So:

• ódýr = cheap
• ódýrari = cheaper

In this sentence it is a predicate adjective after verið, so it describes the subject gallabuxurnar: the jeans had been cheaper.

They are past participles:

• verið = been, from vera
• keypt = bought, from kaupa

Together with hefðu / hefði, they form the verbal phrases:

• hefðu verið = had been
• hefði keypt = would have bought

So the structure is very similar to English in meaning, even though Icelandic uses subjunctive forms more systematically.

Because hettupeysuna is the direct object of keypt, and kaupa takes the accusative case.

For this noun:

• hettupeysan = the hoodie, nominative
• hettupeysuna = the hoodie, accusative

So here Icelandic is marking that the hoodie is the thing being bought.

That is due to Icelandic word order.

When the sentence starts with the ef clause, the main clause usually shows inversion, so the finite verb comes before the subject:

• Ef ..., hefði hún ...

If the main clause stood alone, you would normally say:

• Hún hefði líka keypt hettupeysuna.

This is a very common pattern in Icelandic and is something English speakers often need to get used to.

Líka means also or too.

Here it means that buying the hoodie would have happened in addition to something else already understood from context.

Its position is natural Icelandic adverb placement:

• hefði hún líka keypt hettupeysuna

That placement gives a neutral sense of she would also have bought the hoodie. Moving líka can change the emphasis slightly.

Yes. You can also say:

Hún hefði líka keypt hettupeysuna ef gallabuxurnar hefðu verið ódýrari.

That means the same thing.

The main difference is word order:

• when the ef clause comes first, you get inversion: hefði hún
• when the main clause comes first, normal order is fine: Hún hefði

That is a very common question for English speakers.

In a past counterfactual sentence like this, Icelandic usually prefers hefði keypt rather than myndi hafa keypt.

So:

• hefði keypt = the normal past unreal conditional form here

By contrast, myndi is especially common in present/future unreal conditionals, for example:

• Ef gallabuxurnar væru ódýrari, myndi hún líka kaupa hettupeysuna.
• If the jeans were cheaper, she would also buy the hoodie.

So the sentence you were given uses the standard and natural pattern for an unreal situation in the past.