en_tn/deu/11/21.md

that your days and the days of your children may be multiplied

This can be stated in active form. Alternate translation: "that Yahweh may cause you and your children to live a long time" (See: rc://en/ta/man/jit/figs-activepassive)

to your ancestors

This refers to Abraham, Isaac, and Jacob.

to give them for as long as the heavens are above the earth

This compares how long the people could stay in the land to how long the sky will exist above the earth. This is a way of saying "forever." Alternate translation: "to give them as a possession forever" or "to allow them to live there forever" (See: rc://en/ta/man/jit/figs-simile)