Learn Icelandic: Hvað gerir maðurinn?

In Icelandic, you do not use a helper verb like "does" or "do" to form a question. You just put the main verb directly after the question word. So, hvað gerir maðurinn? literally translates to "What does the man?"

Could I use "hver" instead of "hvað" at the start of this sentence?

No, because hver means "who", while hvað means "what". If you asked Hver gerir þetta? you would be asking "Who does this?" rather than asking what the man is doing.

What is the dictionary form of the verb "gerir"?

The base form (infinitive) is gera, which means "to do" or "to make". Gerir is the conjugated form used for "he", "she", or "it" doing the action right now.

How do we know "maðurinn" means "the man" and not just "a man"?

In Icelandic, the word "the" is attached to the end of the noun as a suffix. The base word for a man is maður, and adding the suffix -inn turns it into maðurinn (the man).