Við hefðum átt að kaupa klósettpappír og sjampó fyrr, því búðin lokaði áður en við komum.

This is the Icelandic way to say should have bought.

Breakdown:

• hefðum = we would have / past subjunctive form of hafa
• átt að = part of the expression eiga að, which means should, be supposed to, or ought to
• kaupa = buy

So við hefðum átt að kaupa means we should have bought.

It expresses hindsight: looking back, the speakers think buying it earlier would have been the right thing to do.

Because hefðum is the past subjunctive, and that fits a meaning like should have or would have.

Compare:

• við höfðum = we had
  This is ordinary past indicative.
• við hefðum = we would have
  This is subjunctive/conditional in meaning.

In this sentence, the speakers are not just stating a past fact. They are expressing regret or an unrealized better choice. That is why hefðum is used.

Yes, but the meaning would shift a little.

• við áttum að kaupa = we were supposed to buy / we had to buy
• við hefðum átt að kaupa = we should have bought

The version in the sentence is more clearly retrospective. It sounds like:

• we did not do it
• now we see that it would have been better if we had

So hefðum átt að kaupa is the better choice for regret after the fact.

They are related, but they do different jobs here.

• fyrr = earlier, sooner
• áður en = before

In the sentence:

• kaupa ... fyrr = buy ... earlier
• áður en við komum = before we arrived

So the sentence is saying:

• we should have bought the items earlier
• because the store closed before we got there

Using both is completely natural.

Áður en is a fixed expression meaning before, when it is followed by a whole clause.

Here:

• áður en við komum = before we arrived

So:

• áður = before
• en = introduces the following clause

It is very common in Icelandic:

• áður en ég fer = before I go
• áður en hún kom = before she came

Yes. Við komum can be either:

• we come (present)
• we came (past)

For the verb koma, those two forms happen to look the same in writing.

In this sentence, the context shows it is past:

• búðin lokaði = the store closed
• therefore áður en við komum must mean before we arrived/came

So Icelandic learners have to rely on context here.

Here, því is a conjunction meaning because.

So:

• því búðin lokaði... = because the store closed...

This use of því is common, especially in written or neutral style. In everyday speech, many speakers would also say:

• af því að búðin lokaði...

Both are correct. The version with því is a little shorter and often feels slightly more formal or written.

Because Icelandic usually puts the definite article at the end of the noun.

So:

• búð = store/shop
• búðin = the store

This suffixed article is one of the most important differences from English.

A few examples:

• hús = house
• húsið = the house
• bíll = car
• bíllinn = the car

So búðin lokaði literally means the-store closed.

Because Icelandic has no indefinite article.

English distinguishes:

• a store
• the store

But Icelandic usually has:

• bare noun for indefinite meaning
• noun + attached article for definite meaning

So:

• búð = a store / store
• búðin = the store

Likewise:

• klósettpappír = toilet paper
• sjampó = shampoo

No separate word like English a is needed.

They are direct objects of kaupa, so they are in the accusative case.

However, in this sentence you do not really see much case change on the surface:

• klósettpappír often looks the same in nominative and accusative singular
• sjampó changes very little as well

So even though the objects are accusative, the forms look simple here.

This is very common in Icelandic: the grammar may be there even when the visible ending does not change much.

Because Icelandic strongly prefers compound words to be written as a single word.

So:

• klósett = toilet
• pappír = paper
• klósettpappír = toilet paper

This is extremely common in Icelandic. English often writes similar expressions as two words, but Icelandic often joins them.

The last part of the compound usually acts as the grammatical head, so klósettpappír behaves grammatically like pappír.

Lokaði is the past tense of loka, meaning to close.

So:

• búðin lokaði = the store closed

It is just a normal simple past form here. The contrast in the sentence is:

• what the speakers should have done: hefðum átt að kaupa
• what actually happened: búðin lokaði

That contrast helps create the feeling of regret or bad timing.