Subtle

едва различимый

(of an argument or concept, words, etc.: requiring one to distinguish between fine points, especially if it is difficult to do so; difficult to grasp; not easily understood or obvious)

неуловимый

neulovimyj

(of an argument or concept, words, etc.: requiring one to distinguish between fine points, especially if it is difficult to do so; difficult to grasp; not easily understood or obvious)

то́нкий

tónkij

(of an argument or concept, words, etc.: requiring one to distinguish between fine points, especially if it is difficult to do so; difficult to grasp; not easily understood or obvious)