Learn Swedish: Den måste besiktigas imorgon. - It must be inspected tomorrow.

This is the s-passive of the infinitive. The active infinitive is besiktiga (to inspect). Because it follows the modal verb måste (must), we must use the infinitive form. To make it passive ("to be inspected"), we simply add an -s to the end of that infinitive to get besiktigas.

Could I use the bli-passive here and say måste bli besiktigad instead?

Yes, den måste bli besiktigad is grammatically correct and means the same thing. However, the s-passive (besiktigas) is highly common and generally preferred for rules, obligations, and official procedures like vehicle inspections. It sounds very natural and concise here.

Why does the sentence use Den for "It" instead of Det?

In Swedish, "it" translates to either den or det depending on the grammatical gender of the noun being replaced. Den tells us that the thing being inspected is an en-word. Given the context of mandatory inspections in Sweden, this sentence is almost certainly referring to bilen (the car).

I often see "tomorrow" spelled as two words. Are imorgon and i morgon both correct?