Learn Croatian: Taj obični dizajn je jako dosadan. - That ordinary design is very boring.

This is the core difference between definite and indefinite adjectives! Obični is definite (ending in -i) because it comes right after the demonstrative word taj (that) — you are pointing out a specific, known design. Dosadan is indefinite (no -i ending) because it comes after the verb je (is) to give new information about the subject. In Croatian, adjectives used to describe a subject after the verb "to be" usually take the indefinite form.

What would happen to the word dosadan if we moved it before the noun, like "that boring design"?

You would need to use its definite form, which is dosadni (taj dosadni dizajn). Notice that the "a" in dosadan disappears! That "a" is a "fleeting a" — it is only there in the indefinite masculine dictionary form to make the word easier to pronounce, but it drops out when you add the definite -i ending.

I learned that jak means "strong" and jako is the neuter form. Why is it translated as "very" here?

You are right that jako is the neuter form of the adjective "strong". However, in Croatian, the neuter adjective form is very often used as an adverb. When used as an adverb before another adjective like it is here, jako translates to "strongly" or, much more commonly, "very".

Could I start this sentence with instead of ?