We do things that make us forget the natural law, or we ignore natural law. But, the knowledge of God's will is inherent in us. You can see why this is such a hot topic of discussion. Prudence, a virtue that commands our actions and judges what we ought to do, has some obvious ties to the idea of natural law. Now prudence is an intellectual virtue, as stated above.
Prudence is a virtue concerned with commanding correct means.
Teaching and experience are needed to instruct one's soul in the correct means. Furthermore, prudence is a virtue, which Aquinas defines as a habit. Habits, as we have stated before, take practice. Just as we break a bad habit by avoiding it, we nurture a good habit by practicing it.