Is Ski laufen together means skiing?
Earlier it was "Ski...fahren"?
So it would never be "Ski gehen" then?
Yes it means "Ski laufen" together, but I would prefer "Ski fahren" sounds better
Does the position of time predicate matter? Is "sie geht Ski laufen manchmal" acceptable?
No, manchmal represents time, which should come right after the verb (in general). Therefore she geht manchmal Ski Lauren.
I wrote "Manchmal geht sie Schifahren" and was marked wrong. Does duo just not like the alternate spelling of "Schi" vs "Ski", or is my sentence actually wrong?