Learn Hardcore Icelandic: Við borðum konfektið eftir kvöldmat.

accusative: time/order/motion along something eftir kv <a class="border-b border-dotted border-blue-400 dark:border-blue-500 cursor-pointer text-inherit no-underline" href="/grammar/icelandic/writing/alphabet">ö ldmat = after dinner (time)
dative: “in pursuit of / longing for / according to” in many common uses Ég sakna hans / Ég þrái eftir honum type patterns (you’ll often see dative forms like honum )

Við here is the 1st-person plural personal pronoun: we.
Be aware that við can also be a preposition meaning by/near/at/against, but in this sentence it’s clearly the pronoun because it’s followed by a verb (borðum).

How do you know borðum means we eat?

The verb is að borða (to eat). In the present tense it conjugates like this:

ég borða = I eat
þú borðar = you eat
hann/hún/það borðar = he/she/it eats
við borðum = we eat
þið borðið = you (pl.) eat
þeir/þær/þau borða = they eat

So borðum is the við (we) form.

Is this present tense, and can it also mean “we are eating”?

Yes, borðum is present tense. Like English, Icelandic present tense can cover both:

a general/habitual meaning (we eat) and
an “in progress” meaning (we are eating)
Context decides. If you want to be extra explicit about “in progress,” Icelandic often uses adverbials like núna (now) rather than a special progressive verb form.