Learn Swedish: Arkeologen började utgrävningen tidigt på morgonen. - The archaeologist started the excavation early in the morning.

You would usually switch to the verb påbörja (to initiate/start) because börja sounds unnatural in the passive. You'd say: Utgrävningen blev påbörjad. Because utgrävning is an en-word, the past participle påbörjad takes the matching en-word ending (it doesn't get a -t or -e).

The word for excavation is utgrävningen. How is this built from the verb "to dig out"?

It comes from the particle verb gräva ut (to dig out). When turning a particle verb into a noun, Swedish flips the particle (ut) to the front, attaches it directly to the verb stem, and adds a noun ending like -ning. So, gräva ut becomes the noun en utgrävning.

Why is it tidigt with a -t at the end instead of just tidig?

Here, tidigt is acting as an adverb describing when the action of starting happened, rather than an adjective describing a noun. In Swedish, you generally add a -t to an adjective to turn it into an adverb, similar to adding "-ly" in English.

Why is morgon in the definite form (morgonen) for "in the morning"?