The subject of the verbs became furious and ground their teeth is not explicit in the Greek; but the Good News Translation has made the subject explicit, the members of the Council, inasmuch as the last previous reference to the Jewish Council appeared in 6.15. As … listened … they became furious attempts to represent the significance of the Greek verb tenses. “As they heard these things” has been expanded to read as the members of the Council listened to Stephen, while in anger has been added to the expression ground their teeth at him in order to indicate for the English reader the significance of this Jewish expression. The idiom ground their teeth at him is quite meaningless in many languages. In fact, the impression may simply be of some ludicrous activity. In some languages, therefore, another idiom with equivalent meaning is employed, for example, “their hearts were very hot against him,” “their faces burned against him,” or “their hearts were very angry.”

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on The Acts of the Apostles. (UBS Handbook Series). New York: UBS, 1972. For this and other handbooks for translators see here .