"Adamo ofte verŝas akvon en glasojn."
Translation:Adam often pours water into glasses.
What you wrote is perfectly understandable to me, in context. If you wish to be a bit more certain of being understood you could also say Adamo ludas muzikon per la glasoj.
It is described in the "tips and notes". Since he is pouring the water into something (the water changes direction), the noun gets a -n-ending. If it did not have a -n-ending the translation would be: "Adam often pours water in glasses." like he is inside the glasses, pouring water (for fun).
Do you understand?
Literally, you can think of it as "Adamo pours water to glasswards" which has the added benefit of making you sound like a pirate!