1. J. H. Tigay, Deuteronomy (JPS Torah Commentary; Philadelphia: Jewish Publication Society, 1996), 59. Christensen's proposal (in Deuteronomy, 2nd ed. (Word Biblical Commentary, 6A; Nashville: Thomas Nelson, 2001), lviii) of a five-part structure—composed of two outer frames (1-5 & 31-34), two inner frames (4-11 & 27-30) and a central core (12-26)—differs from Tigay's only in details and the level of literary sophistication; for him, too, the exposition of the law in 12-26 constitutes the heart of the book.
2. Inward movement refers to the progression A-B-C-D in a symmetrical series A-B-C-D-D'-C'- B'-A', and the outward movement describes the progression D'-C'-B'-A'. Verbal correspondences are underlined.
3. See, e.g., G. von Rad, Deuteronomy: A Commentary (Old Testament Library; Philadelphia: Westminster, 1966), 79; see also A. Phillips, Deuteronomy (Cambridge Bible Commentary; Cambridge: University Press, 1973), 74, and A. D. H. Mayes, Deuteronomy (New Century Bible Commentary; Grand Rapids/London: Eerdmans/Marshall, Morgan & Scott, 1979), 205. Almost all commentaries written for "pastors" and lay-persons have nothing to say about this verse; see, e.g., Ajith Fernando, Deuteronomy (Wheaton: Crossway, 2012), 334-5.
4. The difference in the English translation results from the fact that the form of the verb is Piel perfective in v.12, and Hiphil imperfective in v.26. It is almost impossible to translate the Piel in English; hence the use of an adjective in the expression, "have become corrupt": we have already discussed this verb at 4:25, but here is what we said then:
The verb 'become corrupt' (hiphil of the root shcht) has no English equivalent. The basic meaning of the root is to destroy and is used frequently in the accounts of the flood and of Sodom and Gomorrah to describe both the state of the world and the cities and Yahweh's acts of their destruction (Gen 6:11,112,13,17; 9:11, 15 & 18:28, 31,32; 19:13, 14, 29). 'Become corrupt' thus speaks of behaviour that ensures and invites self-destruction.
The various forms of the verb occur, altogether, 10x in Deuteronomy.
5. See also the overview to Moses' Third Address below.
6. See also the overview to Moses' Third Address below.