Beholdeth (theōrei). Vivid historical present and change from past time.
Opened (aneōigmenon, perfect passive participle with double reduplication, state of completion).
Descending (katabainon). Present active participle describing the process.
Sheet (othonēn). Old word for linen cloth and only here in the N.T. Accusative case in apposition with skeuos (vessel).
Let down (Kathiemenon). Present passive participle of Kathiēmi. Old verb, but in the N.T. only here and Luk 5:19; Act 9:25. Linear action here picturing the process, “being let down.”
By four corners (tessarsin archais). Instrumental case of archē, beginning. We say “end” or extremity for this use of the word. The picture is the sheet held up by four cords to which the sheet is fastened. Isa 11:12 had said that Israel would be gathered from the four corners of the earth. Knowling follows Hobart in taking the four corners of the sheet to be a medical phrase for bandage (the end of a bandage).