Learn Dutch: Ik heb alles, behalve mijn pas. - I have everything, except my pass.

In this sentence, it strictly means "except". When behalve follows absolute words that group things together—like alles (everything), niets (nothing), or iedereen (everyone)—it acts as an exclusion. You will more often see it mean "besides" (in the sense of "in addition to") when it is placed at the very beginning of a sentence.

What kind of "pass" does pas refer to here?

In daily Dutch, a pas (or the diminutive pasje) almost always refers to a small plastic card. It most commonly means a bank debit card (pinpas), a public transit card (OV-chipkaart), or an ID card. The speaker here has likely forgotten their payment card or train pass!

If I start the sentence with Behalve mijn pas, how does the word order change?

Putting Behalve mijn pas at the start of the sentence takes up the first position. This triggers inversion (putting the verb before the subject) in the main clause, making it: Behalve mijn pas heb ik alles. Notice that the comma is dropped, and the meaning shifts slightly toward "Besides my pass, I have everything."

Could I replace mijn pas with dat to say "except that"?