"Exercise" as an uncountable noun (or as a verb) usually refers to the actions you do to improve your health and fitness. But you can also do "an exercise" to develop your skill at something. In your English class, you do the exercises in the book, so here it's a countable noun. "Hardly" is basically a negative, so you're correct that you shouldn't use "some". So, depending on the type of exercise you're talking about: - He hardly does any exercise. - He has hardly done any of the exercises.