8:19 And it shall be, if you forget YHWH your God and go after other gods and serve them and worship them, I solemnly warn you today that you shall perish.

8:20 Like the nations that YHWH makes to perish before you, so shall you perish, because you all would not obey the voice of YHWH your God.

[Helpful parsings]

8:19 [G]. וְהָיָה is here functioning not as a true verb (there is no subject) but as a discourse marker. Read about the discourse function of וְהָיָה in MGBH, §40.A.4 (pp. 327–330) and clarify its significance in the context of Deuteronomy 8.

This function is part of the protasis-apodasis contruction within anticipatory and directive discourse. In this future form, it can be translated “And it will come to pass” or “And it shall be.” The point of this use is to signal something significant such as a turning point or climatic event. The rule to determine whether they are discourse markers is if they introduce protasis-apodasis contruction and lack any clear subject that agrees in number and geneder. Deut. 8:19 is a prime example. The significance of this use in Deuteronomy 8 seems to be that it hightens the warning in verse 19. This warning echoes previous verbs in the passage that warn Israel to be careful to do the whole commandment and not forget YHWH (vv. 1, 6, 11, 14, 17-18). It now culminates with this discourse marker, rendering an emphasis in translation: “I solemnly warn you.”