God puts vanilla and chocolate ice cream in front of a man and tells him that he will choose chocolate. Can the man choose vanilla?
Your answer is?
Sure, the man can choose vanilla
Good answer.
unless God forces him to eat the chocolate.
Well that's the entire crux of the matter, isn't it? If God infallibly knows that the man will choose chocolate, but the man chooses vanilla, then that either means that God's knowledge ISN'T infallible, or it means that God lied to the man about him choosing chocolate while knowing that he would instead choose vanilla.
On the other hand, if God's knowledge IS infallible, and God wasn't lying to the man, then the man will have no choice BUT to choose chocolate.
The formal argument goes something like this:
T= the man will choose chocolate ice cream
(1) When God presented the ice cream to the man, God infallibly believed T. [Supposition of infallible foreknowledge]
(2) If E occurred in the past, it is now-necessary that E occurred then. [Principle of the Necessity of the Past]
(3) It is now-necessary that God believed T. [1, 2]
(4) Necessarily, if at the moment God believed T, then T. [Definition of “infallibility”]
(5) If p is now-necessary, and necessarily (p → q), then q is now-necessary. [Transfer of Necessity Principle]
(6) So it is now-necessary that T. [3,4,5]
(7) If it is now-necessary that T, then the man cannot do otherwise than choose the chocolate ice cream. [Definition of “necessary”]
(8) Therefore, the man cannot do otherwise than choose the chocolate ice cream. [6, 7]
(9) If you cannot do otherwise when you do an act, you do not act freely. [Principle of Alternate Possibilities]
(10) Therefore, when the man chooses the chocolate ice cream, he will not do it freely. [8, 9]
In addition, he will not be able to choose the vanilla, because that is contrary to T.