Röðin í matvöruversluninni var löng á föstudag, svo ég fór heim.

Röðin is röð (line/queue) with the definite article attached as a suffix: röð + -in → röðin = the line.
It’s also in the nominative singular because it’s the subject of the clause (Röðin … var …).

Röð is feminine. Gender matters because it affects:

• the definite ending: feminine nominative singular often uses -in (as in röðin)
• adjective agreement: löng is feminine nominative singular to match röðin

í can govern either:

• accusative (movement into), or
• dative (location in/inside)

Here it’s location, so it takes dative: í …-inni = in the (specific) grocery store.
matvöruversluninni is matvöruverslun + definite dative singular ending -inni.

It’s not “double definite” like in some languages; it’s simply that both nouns are made definite because the speaker is referring to specific things:

• Röðin = the line
• matvöruversluninni = the grocery store (that we’re talking about)

You could make one or both indefinite depending on context, e.g.:

• Röð í matvöruverslun var löng = a line in a grocery store was long (more general/less natural as a standalone)

Yes—it's a common Icelandic compound:

• mat- = food
• vöru- = goods/products (genitive/plural sense used in compounds)
• verslun = store/shop

So matvöruverslun literally ≈ food-goods-store → grocery store.

var is the past tense (preterite) of vera (to be).
So Röðin … var löng = The line was long.

Present would be:

• Röðin … er löng = The line is long.

Adjectives agree with the noun in gender, number, and case.

• röð is feminine singular nominative
• therefore the adjective is feminine singular nominative: löng

The base adjective is typically listed as langur (masc.), with forms:

• masculine: langur
• feminine: löng
• neuter: langt

Both exist, with a nuance:

• á föstudag = on Friday (often general/at-a-time reference; very common)
• á föstudaginn = on the Friday / this past/that specific Friday (more explicitly definite)

In everyday speech, á föstudag is often used even when you mean a specific recent Friday, but -inn makes the specificity clearer.

Here svo functions as a conjunction meaning so / therefore, linking cause → result:

• …, svo ég fór heim. = …, so I went home.

svo can also mean then in other contexts, but in this sentence it’s the causal so.

Both can be possible depending on style and structure, but here svo is used like a coordinating conjunction, so the next clause often keeps normal word order:

• svo ég fór heim (subject + verb)

If you start a new sentence or want a slightly different emphasis, you may also see:

• Svo fór ég heim. = So then I went home. (often “then/so” as an adverb-like starter)

fór is the past tense of fara (to go / to travel):

• present: ég fer = I go
• past: ég fór = I went

So ég fór heim = I went home.

In Icelandic, heim works as an adverb meaning home (wards), similar to English go home (also no preposition).
So you say:

• fara heim = go home
  Whereas heima is the “at home” form:
• vera heima = be at home

A few common ones:

• ð in föstudag is like a soft “th” in this (but can be very light)
• ö in Röðin / löng is a rounded vowel (not like English “o”)
• Stress is usually on the first syllable: RÖ-ðin, MAT-vöru-verslun-inni, FÖS-tu-dag
• ll/nn spelling doesn’t automatically mean English-style “clear L”; focus more on vowels and the rhythm first.