en_tn/jer/31/37.md

Only if the highest heavens can be measured, and only if the earth's foundation below can be discovered, will I reject all of Israel's descendants

Yahweh speaks of situations that will never happen to assure his people that he will never reject all of Isreal's descendants. Alternate translation: "Just as the highest heavens can never be measured, and the earth's foundation below can never be discovered, I will never reject all of Israel's descendants" (See: rc://en/ta/man/jit/figs-hypo)

the highest heavens ... earth's foundation

These phrases refer to the whole of creation. (See: rc://en/ta/man/jit/figs-merism)

the earth's foundation below can be discovered

This can be translated in active form. Alternate translation: "someone can dig all the way down to the earth's foundation" (See: rc://en/ta/man/jit/figs-activepassive)

foundation below

"deepest part." The "foundation" is the part of a house on which every other part rests.

reject

throw away or refuse to take

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/jit/figs-123person)