Learn Icelandic: Hvenær hittumst við?

The ending -st changes the base verb hitta (to meet) into the middle voice. While hitta means to meet someone and requires a direct object (like "I meet him"), the -st version adds an "each other" meaning. So við hittumst means "we meet each other."

Can I just use the base verb and say Hvenær hittum við?

No, that would sound incomplete. Because the base verb hitta needs an object, a sentence like Hvenær hittum við? would leave the listener waiting to hear who you are meeting. You have to use the -st form to just say "When do we meet up?"

How is the -st form created for the pronoun við (we)?

It is very straightforward for the "we" form! You just take the regular present tense form for við, which is hittum (we meet), and add -st directly to the end to get hittumst.

Why does the verb come before við in this sentence?

This is a quick reminder of the V2 (verb-second) rule. In Icelandic questions that start with a question word like hvenær (when), the verb must take the second slot. So the verb hittumst swaps places with the subject við to stay in that second position.