Learn Swedish: Den unga Anna tyckte att filosofi var tråkigt. - Young Anna thought that philosophy was boring.

This is an example of categorizing neuter. When describing an abstract concept or general field of study like filosofi, Swedish uses the neuter adjective form (tråkigt) even though the noun is an en-word (en filosofi). It functions as if you are stating that the general concept of philosophy is a boring thing.

Why do we need Den before unga Anna? Doesn't a name already count as definite?

In Swedish, when you put an adjective in front of a proper noun like a name, you normally must include the independent definite article (Den or Det). While the English translation does not use an article, Swedish requires Den unga Anna to make the grammatical structure complete.

Why is the adjective spelled unga and not ung?

Because the phrase is definite and uses the article Den, the adjective has to take its weak form, which ends in -a. You must use this weak form anytime an adjective follows den, det, or de.

Could I use tänkte or trodde instead of tyckte to mean thought here?