Learn Swedish: Han var stressad när branden började. - He was stressed when the fire started.

när (when) starts a subordinate clause, which is a supporting part of the sentence. Inside this clause, the subject (branden) comes right after the conjunction, followed by the verb (började). Since this whole clause is at the very end of the sentence, it doesn't change the normal subject-verb order of the main part (Han var).

What happens to the word order if we start the sentence with the när-clause instead?

If you put the när clause at the beginning, it counts as the first block of information. Because Swedish places the verb in the second position of a main clause (the V2 rule), the main verb (var) must come immediately after the clause, before the subject (han). The sentence would become: När branden började var han stressad.

Is stressad considered a verb or an adjective here?

It acts as an adjective describing his state of mind. It is built from the verb att stressa (to stress), but the -ad ending turns it into a describing word. Because it acts like an adjective, it must agree with the subject: if it were "they were stressed," it would change to the plural form: De var stressade.

Why does the word brand have an -en at the end?

In Swedish, the definite article (the) is attached to the end of the noun. Adding -en to the en-word brand (fire) makes it branden, which means "the fire".