Learn Dutch: Ben vanmiddag helaas verhinderd. - Am unfortunately unable to make it this afternoon.

In casual spoken Dutch, or in quick written messages like texts and emails, the subject is often dropped if it's at the very beginning of the sentence and easily understood from context. Here, Ik (I) is omitted. The full sentence would be Ik ben vanmiddag helaas verhinderd.

Verhinderd looks like a past participle. How is it being used here?

It is the past participle of verhinderen (to prevent or hinder), but it functions here as an adjective. Verhinderd zijn is a standard, polite way to say you are "unable to attend," "tied up," or "unavailable" for a scheduled appointment or meeting.

If dropping the subject is casual, isn't it weird to use a polite word like verhinderd in the same sentence?

Not in Dutch workplace culture! This mix is very common in professional texts or quick emails. Dropping Ik makes the message brief and efficient, while using verhinderd maintains a polite, professional boundary (meaning you don't have to overshare exactly why you can't make it).

Could I swap the adverbs and say Ben helaas vanmiddag verhinderd?

You can, but the original word order is more natural. In Dutch, time phrases (vanmiddag) generally come before other adverbs or expressions of manner (helaas). Putting helaas first isn't strictly wrong, but it places heavy, somewhat dramatic emphasis on the word "unfortunately."