Learn Swedish: Föreställningen skulle börja utomhus innan det började åska. - The performance was going to start outdoors before it started thundering.

Skulle is the past tense of ska (will / are going to). When you pair it with a bare infinitive like börja (to start), it expresses the "future in the past". We use this to talk about things that were planned or expected to happen, just like saying "was going to" or "was supposed to" in English.

Why is the verb for "start" used in two different forms (börja and började)?

The first one follows the auxiliary verb skulle, so it must be in the bare infinitive form: börja. The second one describes the action that actually happened and interrupted the plan, so it acts as a normal main verb in the simple past tense: började (started).

Why isn't there an att (to) between började and åska?

Here, åska is used as a verb in its infinitive form, meaning "to thunder". After the verb börja (to start), the infinitive marker att is completely optional. Saying började åska is very common and natural, though började att åska is also grammatically correct.

I know utomhus means "outdoors". What is the word for "indoors"?