Learn Swedish: På sommaren är vi ofta utomhus. - In the summer we are often outdoors.

To talk about something you do as a habit during a certain season (like "in the summer" or "during summers generally"), Swedish uses the preposition på followed by the definite form of the season. That's why sommar becomes sommaren.

Why is the word order är vi instead of vi är?

This is the famous Swedish V2 (verb-second) rule! Because the sentence starts with the time phrase På sommaren, the verb (är) must take the second position in the sentence. This pushes the subject (vi) right after the verb.

If På sommaren means "in the summer" as a general habit, how would I say "this summer"?

If you want to talk about this specific summer rather than a seasonal habit, you don't use på or the definite ending. Instead, you would use i sommar for "this coming summer" or i somras for "this past summer".

Could I just say ute instead of utomhus?

Yes! Both mean "outside" or "outdoors". Utomhus is a bit more specific to being physically outside of a building (it literally means "outside-house"), while ute is the shorter, more general word you will hear very often in daily conversation.