“very courageous.” The “very” could also be put before “strong” such that the phrase read, “be very strong and courageous.” But since in the Hebrew text the word “very” is before “courageous,” most translations read that way.

“Law.” The word “Law” is “Torah.” Here we see that the Book of Joshua, the first book in the Prophets in the Hebrew Bible, begins with a reminder to be anchored in the Torah. Studying the Torah, the first five books of the Bible, is essential to learn how to think and reason the way God does. [For more on the meaning of Torah, see commentary on Proverbs 1:8].

“have success.” The verb can mean “have success” (HALOT Hebrew and Aramaic Lexicon of the Old Testament) but commonly it has to do with having understanding or prudence; and that gives good decision-making ablility, which leads to success.