-"Inn" can be used in an imperative sentence such as "Gå inn!" ="Go inside!" -"Inne" basically means inside. "Det er varmt her inne" = It's warm in here" -"i" can be used after "inne" to describe placement inside something. "Det er inne i esken" = "It's inside the box"
-"i" can be translated to "in". "Det står i boka". ="It's written in the book" "De bor i Norge" = "They live in Norway" "I vår tid." = "In our time."
-To connect two identical nouns: "De bor vegg i vegg" ="They live next door" (lit: they live wall in wall)
-To indicate means etc: "Jeg sa det i beste mening" = "I said it with the best intentions"
-"På" can indicate placement: "Det står på bordet" =It's on the table"
-To describe means, reason: "Han er på flukt" = He's on the run"
-With a verb: "Du ligner på din mor" ="You look like your mom"
...And much more. You'll get a feeling of it eventually :) And I'm sure the course will go into more detail on this as well.
So do we use "sitter på stolen" if we are talking about a normal kind of chair, like a kitchen chair e. g. because than you really are on top of the thing, and "sitter i stolen" if we are referring to a kind of armchair (this one-person-sofa-thingy) because than the chair is literally around you? Can "stol" refer to both of this objects or do you have to use lenestol for the latter?