en_tn_condensed/mrk/09/18.md

he foams at the mouth

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: "bubbles come out of his mouth"

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)