en_tn_lite_do_not_use/deu/32/46.md

He said to them

"Moses said to the people of Israel"

Fix your mind on

This is an idiom. Alternate translation: "Pay attention to" or "Think about" (See: rc://en/ta/man/translate/figs-idiom)

I have witnessed to you

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.

your children

"your children and descendants"