Learn Icelandic: Skilið verkefninu í dag! - Hand in the assignment today!

The ending -ið tells us this is a plural command. The infinitive (dictionary) form of the verb is skila (to return or hand in). If you were speaking to just one person, you would use the singular command form skilaðu.

Why does verkefninu end in -nu here instead of -ð?

The ending -nu does two things: it adds the word "the" to the neuter noun verkefni (assignment), and it shows the dative case. The verb skila forces the object being handed in to take the dative case, so we must use verkefninu instead of the basic form verkefnið.

Is it normal to leave out the word for "you" (þið) in this sentence?

Yes! In Icelandic plural commands, the pronoun þið (you, plural) is usually dropped completely. The -ið ending on Skilið already makes it perfectly clear who you are giving the instruction to.

I learned that the preposition í usually takes the dative case for location. Why is dag in the accusative case here?

While í takes the dative case when talking about being physically located inside something, í dag (today) is a fixed phrase. When expressing time, Icelandic prepositions frequently use the accusative case instead.