If I terrastalised my Pokemon into a ghost type and used curse, would the opponent be cursed?
Clodsire has Terastallized into the Ghost-type! (...) Clodsire used Curse! Clodsire cut its own HP and put a curse on the opposing Annihilape! (Clodsire lost 49.9% of its health!) The opposing Annihilape is afflicted by the curse!
Replay
Curse works as it would with an original Ghost-type if you terastallize into Ghost-.