Prepositions/Adverbs

[blouen]

I'll have a report on Prepositions overlapping with adverbs. Could you please explain how this happens. How do I distinguish one from the other?


* {down the stairs} / downstairs
* {under the ground} / underground
* inside {the house} /
* aboard {the plane}
* underneath {the surface}

[Soup]

Prepositions need an object, adverbs don't.