I visited the Folksmuseum in Oslo last month, after completing this course. While there, I met a student who was studying architecture and norwegian culture. He chatted to my norwegian friemd about how he was taking steps towards his goal of designing buildings inspired by norwegian cultural elements. I could pick up parts of the conversation, but managed to surpise them both when I casually used this phrase.
Thanks Duolingo team! This whole course was worth it for that moment
"til" is not mandatory. I don't think there's a rule for when to omit it, sometimes it sounds natural and other times it's a bit 'too much'. "Drømmen blir til virkelighet" sounds fine with "bli+til", whereas "jeg blir dommer" would work better without "til" (even though both are correct).
I know it's a one year old question, but I believe a good hint would be to say tha if you could replace "become" with "turn in to" you can use " bli til", whereas when this sounds strange in englis, it would be strange to use "bli til" in norwegian. When you becoma a judge it would be strange because you take on a role as one, and you don't go through a metamorphosis and turn in to one. Other examples: "natt blir til dag", "år blir til dag", "et barn blir til en voksen", "en marmorblokk blir til et kunstverk", but "et barn blir elev", "en elev blir student", "en mann blir pilot".