i OR på
The words i and på can seem to be used without meaning in the beginning of a Danish language journey, and often they don’t correspond so well with English. That being said, here is one clear distinction between the two when talking about the language something is in, that is, a movie, book etc.
På | The language something is in
Anytime you want to say a book, movie, or song etc. is in Danish you have to you på. This may seem counterintuitive due to saying in, in English, but in the words of Tupac, that’s just the way it.
Bogen er på dansk | The book is in Danish
Jeg ser en film på dansk | I’m watching a film in Danish
Vi snakker sammen på dansk | We’re chatting in Danish
i | The subject you teach
i is only used with language when talking about the language you teach, or any other subject in which you teach.
Jeg underviser i dansk | I teach Danish
Min far underviser i psykologi | My dad teaches psychology
You can however say:
Jeg underviser på dansk | I teach in Danish
But this refers to the language you speak when you are teaching, that is, you are speaking in Danish, but could be teaching another subject.
So another example could be:
Jeg underviser i guitar på dansk | I teach guitar in Danish