He supposed (ενομιζεν). Imperfect active of νομιζω. He was supposing, Stephen explains, when he smote the Egyptian.

That his brethren understood (συνιενα τους αδελφους). Present active infinitive of συνιημ, to send (put) together, to grasp, to comprehend, in indirect discourse with the accusative of general reference.

By his hand was giving them deliverance (δια χειρος αυτου διδωσιν σοτηριαν αυτοις). Picturesque use of "hand" as in Acts 2:23, present active indicative of διδωμ retained in indirect discourse after imperfect ενομιζεν. But they understood not (ο δε ου συνηκαν). Page notes "the rhetorical power of these words" from Stephen. Συνηκαν (first aorist indicative, κ aorist) refers to συνιενα just before.