Possible meanings are 1) "I have testified to you," referring to what Yahweh has said he will do to the Israelites if they disobey, or 2) "I have commanded you," referring to what Yahweh was commanding them to do.
This litotes can be stated positively. AT: "something very important" (See: [[rc://en/ta/man/translate/figs-litotes]])
# because it is your life
The abstract noun "life" can be translated as the verb "live." You may need to make explicit the metonymy of obeying the law, which represents the law itself. AT: "because you will live if you obey it" (See: [[rc://en/ta/man/translate/figs-metonymy]])
Long days are a metaphor for a long life. See how you translated these words in [Deuteronomy 4:26](../04/25.md). AT: "be able to live a long time" (See: [[rc://en/ta/man/translate/figs-metaphor]])