OK, why did God allow Adam to partake of the tree of knowledge? Could he have prevented it? If so, why didn't he?There is no such thing as "passive decree".
It is a modern philosophical invention that Calvinists attempt to avoid the direct statement of Calvin himself, to wit:
..." God ...determined whom he would admit to salvation, and whom he would condemn to destruction".
The Westminster Confession says God "freely and unchangeable ORDAINED whatsoever comes to pass". By definition and logically that would include sin, unbelief and damnation.
"Passive decree" contradicts Theological Determinism which Calvinists themselves affirm. If every event is predetermined then the damnation of the reprobate is not 'passively allowed' it is actively determined.
Calvinism says the reprobate CANNOT believe unless God gives irresistible grace. Withholding that grace is therefore an ACTIVE determination of their damnation.
(Claiming God decreed for Adam to sin is identical to Mormon doctrine that the Fall MUST happen so that redemption may come)
And Why did God place that tree in the garden to begin with?
Does God ordain all things that come to pass or was God "taken by surprise" when Adam disobeyed him?