Breakdown of Við vorum að grafa fyrir þremur vikum síðan.
vera
to be
við
we
að
to (infinitive marker)
þrír
three
Questions & Answers about Við vorum að grafa fyrir þremur vikum síðan.
How does the phrase fyrir þremur vikum síðan translate to "three weeks ago"?
To say "ago" in Icelandic, you use a two-part phrase: fyrir + [amount of time] + síðan. It acts like a frame around the time. This literally translates to "before [time] since", but together the whole phrase simply means "three weeks ago".
Why do we say þremur vikum instead of the basic þrjár vikur?
In the fyrir ... síðan structure, the preposition fyrir forces the time amount into the dative case. The feminine plural word for weeks, vikur, becomes vikum. The number three must also match the case and gender, so þrjár changes to its dative form, þremur.
Why use vorum að grafa instead of a single past tense verb?
Just like in English, Icelandic has a continuous tense to show that an action was ongoing in the past. You use the past tense of vera (to be) — which is vorum for við (we) — plus að and the infinitive verb (grafa). It matches the English "were digging" perfectly.
Do numbers usually change their form like þremur does here?