en_tn_condensed/mrk/09/18.md

down and he foams at the mouth, grinds his teeth

A convulsion, or seizure, can cause a person to have trouble breathing or swallowing. This causes white foam to come out of the mouth. If your language has a way to describe that, you could use it. Alternate translation: "down, and he has bubbles come out of his mouth and grinds his teeth"

he becomes rigid

"he becomes stiff" or "his body becomes rigid"

they could not

This refers to the disciples not being about to drive the spirit out of the boy. Alternate translation: "they could not drive it out of him" (See: rc://en/ta/man/translate/figs-ellipsis)