Í gær var slydda og rok, svo við fórum ekki lengra en út í búð.

Í gær is the normal Icelandic expression for yesterday.

It is a fixed time expression:

• í originally means in
• gær is an old word connected with yesterday

As a learner, the safest thing is to treat í gær as one chunk meaning yesterday.

English usually needs a dummy subject in weather sentences, as in it was raining.

Icelandic often does not need that. In weather or general-condition statements, Icelandic can simply say:

• var slydda og rok without any equivalent of English it.

So this is normal Icelandic, not an omission.

This is common in Icelandic weather-style or presentational sentences.

Here, slydda og rok is being presented as one overall weather situation, so singular var sounds natural.

A more literal subject-first structure could make plural more likely in some contexts, but in a sentence like this, singular var is very idiomatic.

• slydda = sleet, or wet snowy/rainy slush falling from the sky
• rok = strong wind, gale

So together they describe nasty weather conditions.

Here svo means so or therefore.

It connects the two clauses like this:

• bad weather happened
• so we did not go any farther

In other contexts, svo can also have meanings like then, thus, or as, but here it is clearly a result word: so.

Because ekki usually comes after the finite verb in a main clause.

So Icelandic normally says:

• við fórum ekki = we did not go

This is a very common pattern:

• ég veit ekki = I do not know
• hann kemur ekki = he is not coming

For English speakers, this is worth getting used to early.

Fórum is the 1st person plural past tense of fara, which means to go.

So:

• fara = to go
• við förum = we go
• við fórum = we went

It is an irregular verb, so the stem changes in the past.

It means not farther than or no farther than.

So:

• við fórum ekki lengra en ... = we did not go any farther than ...

This structure is very natural in Icelandic:

• lengra = farther/further
• en = than

Because lengra is used for distance.

Here the sentence is about how far they went, so lengra is the right form.

Compare:

• lengra = farther, further in distance
• lengur = longer, any longer, no longer, usually about time or continuation

For example:

• ekki lengra = not farther
• ekki lengur = no longer

That distinction is very important.

Yes, it is the same written word, but here it means than.

After a comparative such as lengra, en means than:

• lengra en út í búð = farther than out to the shop

In other sentences, en can mean but:

• Ég fór, en hann var heima = I went, but he stayed home

So the meaning depends on context.

Út í búð is a very common Icelandic expression meaning out to the shop/store.

• út adds the sense of going out
• í búð gives the destination

So fara út í búð is much like English go out to the store.

You can also hear fara í búð, which is also common, but út í búð often sounds especially natural when someone is leaving home to go to the store.

Because Icelandic often leaves out the article in set expressions like fara í búð or fara út í búð.

In English, we often say go to the shop/store, but Icelandic can use the bare noun in this kind of general, familiar destination phrase.

So:

• út í búð = to the shop / to the store
• út í búðina would sound more like a specific, particular shop

Both are possible, but the article-less version is very common and idiomatic.

It is accusative, because í takes:

• accusative for motion into/toward a place
• dative for being in a place

Here there is movement:

• fórum ... í búð = went to/into the shop

So accusative is expected.

For this noun, the accusative singular looks the same as the nominative singular:

• búð

That is why you do not see any visible ending change here.