A.T. Robertson Commentary Acts 4:31

"And when they had prayed, the place was shaken wherein they were gathered together; and they were all filled with the Holy Spirit, and they spake the word of God with boldness." — Acts 4:31 (ASV)

The place was shaken (εσαλευθη ο τοπος). By an earthquake most likely as in 16:26, but none the less a token of God's presence and power (Psalms 114:7; Isaiah 2:19,21; Hebrews 12:26f.).

Were gathered together (ησαν συνηγμενο). Periphrastic past perfect passive of συναγω.

They spake (ελαλουν). Imperfect active indicative, began to speak, after being filled (επλησθησαν, aorist passive indicative) with the Holy Spirit. Luke uses the very words of the prayer in verse 29 to describe their conduct.