Nauseous = adjective = descriptive word = ability to make one sick to one’s stomach
The roller coaster ride was nauseous.
The oozing rash is nauseous.
Nauseated = verb = action word = having been made sick to your stomach
I was nauseated after riding the roller coaster.
I was nauseated after treating the oozing rash I got at summer camp.
Use the right word and I promise to not correct you under my breath in the presence of mixed company. My doing so may be nauseous and make you nauseated.