"The forest" in English is a collective plural of trees, equivalent in meaning to "the woods". If one were to say "the forests" one would mean more than one set of woods, as in "Canada has many forests in all of its provinces." Were I to be going into one set of trees only (ie the woods) I would be going into "the forest" not "the forests".
"Inn i skogene" is very strange in Norwegian.
If you google-translate "into the woods" to Norwegian, the translation is: "inn i skogen".
However, "Into the woods" is the name of an American musical with Meryl Streep. Of course, if you don't know this, this exercise will be confusing.
I didn't know so my tranlation was of course, being Norwegian, "inn i skogen".
You might have noticed that this happens quite often; the translation is not correct Norwegian, because the exercise is something from a song or the name of a book.
I've never experienced this in the French or the Chinese lessons.