Breakdown of Voruð þið á ströndunum?
vera
to be
þið
you (plural)
á
onto / on / in (static location) / on (a day of the week)
ströndin
the beach
Questions & Answers about Voruð þið á ströndunum?
Why does the word for beach end in -unum here instead of just -um?
This is the definite dative plural! The basic dative plural ending is -um (ströndum means "beaches"). When you want to say "the beaches", you use the definite dative plural ending -unum, making it ströndunum.
Why do we use the dative case for ströndunum in this sentence?
The preposition á (meaning "at" or "on" here) forces the following noun into the dative case when it describes a fixed location. Since you are asking if they were hanging out at the beaches (rather than moving towards them), it takes the dative.
Why do we use Voruð instead of Varst?
Varst is the past tense used for þú (singular "you"). This sentence uses þið, which is the plural "you" (like "you all"). Voruð is the correct past tense form of vera (to be) that matches þið.
Why does the sentence start with the verb Voruð?
To form a yes/no question in Icelandic, you simply swap the subject and the verb. Instead of the statement Þið voruð... (You were...), you flip it to Voruð þið...? (Were you...?).