Learn Icelandic: Heyrir þú okkur?

Great memory! The word okkur (us) is actually exactly the same in both the accusative and the dative case. In this sentence, it is acting as the accusative because the verb heyra (to hear) always takes an accusative object.

Where is the word for 'Do' in this sentence?

Icelandic doesn't use helper verbs like 'do' or 'does' to make questions. Instead, you just swap the subject and the verb. So the statement 'You hear us' (Þú heyrir okkur) simply becomes the question 'Hear you us?' (Heyrir þú okkur?).

Why does the verb heyra end in -ir here?

The verb changes its ending to match the subject. Because the subject is þú (you, singular), the correct form is heyrir. If the subject were 'I' (ég), it would just be heyri.

If I wanted to ask a group of people 'Do you hear us?', how would this sentence change?

You would change the subject from þú (singular 'you') to þið (plural 'you') and update the verb to match. The new sentence would be Heyrið þið okkur?.