Błędnie: I can't understand why he's so neurotic today. There must be something I don't know about. Poprawnie: I can't understand why he's so nervous today. There must be something I don't know about.
"Neurotic" to 'neurotyczny', 'nerwowo chory' 'chronicznie nerwowy'. "Nervous" to 'nerwowy', 'zdenerwowany'.