Why did God place the tree of knowledge of good and evil in the garden?
This is a good question. Of course, we do not have a direct answer to this question, either in Genesis or anywhere else in the Bible. Yet, I believe that from what we know of God elsewhere in the Bible we can make a very probable guess. It is about free will. God gave the entire garden to Adam and Eve. They had a great relationship with him. God’s intention was that Adam and Eve would love him and he would love them and their children. The think about love is that live requires a choice. Love cannot be forced. Therefore, Adam and Eve had to have a choice to either love God or to not love God. This "knowledge" included knowledge of forbidden things. Eating of this tree opened a "Pandora’s Box" of possible evil choices. Bottom line, God’s plan is love, but love requires a choice. God gave Adam and Eve a real choice. Not just a symbolic one. When they chose evil, evil came into the world and, unfortunately we all have followed the example of Adam and Eve. We, too, have eaten of this fruit, only to a much greater extent than did Adam and Eve that day in the garden. I believe that the story of Adam and Eve in the garden represents what really happened. Whether it is partially symbolic of what happened or a realistic account is not really the point. The main point is that Adam and Eve represent all of us who, as accountable adults have done, which is to rebel, reject God’s will and chosen to sin–therefore losing our innocense.