Foreknew is foreknew, predestinate is predestined. He foreknew them, meaning He did something(chose/elected them) in foreknowing them.
Both terms are the "why" of an explanation to children, but not the "how" of all truth for the mature. They both are like God as a parent explaining to a young child
how going to the doctor or dentist is going to go and
how that its okay, without going into the details of
why that only an older child would be able to relate to and rationalize.
First the natural, then the spiritual.
But the natural merely points to the spiritual. Much of what surrounds God's dealings with the natural become obsolete with those who are born of the spirit...including the old doctrine that pointed the way. This is what Paul meant by leaving behind the elementary principles of Christ. Such is not against Christ or the scriptures, but for Christ, and for the fulfillment of the scriptures.