Plainly is implied. The word translated show does not by itself suggest supernatural revelation; the writer is simply explaining what he thinks the words “once more” imply. The words “once more” plainly show may be rendered as “Those words ‘once more’ indicate clearly” or “… surely mean.”

The created things may be rendered as “that which has been created” or “what God has created.” The created things will be shaken and removed is literally “(the) removal of things-shaken as having-been-made.” Good News Translation gives the meaning. The word translated removed was adequately translated changed in 7.12, but in the present context that would be too weak. Here the meaning is “set aside,” or “carried away,” or “be replaced by something else.” The text implies that earthly things can be shaken and removed because they are “made”; the same idea was expressed in an Old Testament quotation in 1.10-12. On the other hand, things which belong to heaven, not to the visible world, cannot be shaken (literally, “are not shaken,” but Good News Translation is clearer).

Quoted with permission from Ellingworth, Paul and Nida, Eugene A. A Handbook on The Letter of the Hebrews. (UBS Handbook Series). New York: UBS, 1983. For this and other handbooks for translators see here .