A.T. Robertson Commentary Acts 12:16

"But Peter continued knocking: and when they had opened, they saw him, and were amazed." — Acts 12:16 (ASV)

Continued knocking (επεμενεν κρουων). Imperfect active and present participle. Now all heard the knocking.

When they had opened (ανοιξαντες). First aorist active participle of ανοιγω or -νυμ. The whole group rushed out to the courtyard this time to make sure.

They were amazed (εξεστησαν). The frequent second aorist active (intransitive) indicative of εξιστημ.