en_tn/jer/31/36.md

Only if these permanent things vanish ... will Israel's descendants ever stop from forever being a nation

This is a hypothetical situation that will never happen. Yahweh says this to assure the people of Israel that they will always be a nation. Alternate translation: "These permanent things will never vanish ... and Israel's descendants will never stop from forever being a nation" (See: rc://en/ta/man/translate/figs-hypo)

this is Yahweh's declaration

Yahweh speaks of himself by name to express the certainty of what he is declaring. See how you translated this in Jeremiah 1:8. Alternate translation: "this is what Yahweh has declared" or "this is what I, Yahweh, have declared" (See: rc://en/ta/man/translate/figs-123person)