When they heard (akouontes). Present active participle of akouō, while hearing.
They were cut to the heart (dieprionto tais kardiais). See note on Act 5:33 where the same word and form (imperfect passive of diapriō) is used of the effect of Peter’s speech on the Sadducees. Here Stephen had sent a saw through the hearts of the Pharisees that rasped them to the bone.
They gnashed on him with their teeth (ebruchon tous odontas ep' auton). Imperfect (inchoative) active of bruchō (Attic brukō), to bite with loud noise, to grind or gnash the teeth. Literally, They began to gnash their teeth at (ep') him (just like a pack of hungry, snarling wolves). Stephen knew that it meant death for him.