"She has put the pillow on the couch."
Translation:Hon har lagt kudden i soffan.
When the sofa is seen as enclosing you, use i. When the sofa is seen as just under you, use på. English does the same with chairs, you can sit in a chair if you think of it as enclosing you, but on it if the idea is more that you're on top of it, but they only say on about sofas, couches, settees etc.