Albert Barnes Commentary Mark 9:18

"and wheresoever it taketh him, it dasheth him down: and he foameth, and grindeth his teeth, and pineth away: and I spake to thy disciples that they should cast it out; and they were not able." — Mark 9:18 (ASV)

And wherever. In whatever place, at home or abroad, alone or in public.

He tears him. He rends, distracts, or throws into convulsions.

He foams. At the mouth, like a mad animal. Among us, these would all be considered as marks of violent derangement or madness.

And pines away. He becomes thin, haggard, and emaciated. This was the effect of the violence of his struggles, and perhaps of the lack of food.