* Simple description: His forgetfulness made it difficult to keep track of appointments.
* Figurative language: The fog of war had settled upon his mind, leaving him in a state of forgetfulness.
* Emphasizing impact: The sheer extent of his forgetfulness was alarming.
* With a specific example: Her forgetfulness sometimes led her to leave the stove on.
* As a characteristic: Forgetfulness was a common trait among the elderly.
Which sentence best fits your needs depends on the context you want to create!