"My favorite subject at school is history."
Translation:Moim ulubionym przedmiotem w szkole jest historia.
Because "historia" is subject , "Moim ulubionym przedmiotem" is object, and technically it's "historia jest moim ulubionym przedmiotem w szkole "
It sounds better the way defined translation is, but my translation should be accepted too, if it is not then report.
Also you can use "to" here and not worry about instrumental.
No. You cannot define "My favourite school subject" as being "history". You can go the other way round, define "history" as being "my favourite school subject" and say "Historia jest moim ulubionym przedmiotem w szkole". But your version doesn't work.
See Part 1,5 here: https://www.duolingo.com/comment/16373167
I'm a bit confused by your question. Do you mean that "Przedmiot jest historią" sounds wrong but "Ten przedmiot jest historią" is okay? Yeah, that's probably what you meant.
I would say it this way: imagine that the right side of the sentence has an indefinite article. So for example "Paul is a doctor" makes perfect sense, but "The doctor is a Paul" makes none. Similarly, "History is a subject" is ok. But "The subject is a history" not really, at least not in this meaning and context :)