Sin is defined as "breaking the laws of God" (Gospel Principles, 1997, page 382). "Some believe Adam and Eve committed a serious sin when they ate of the tree of knowledge of good and evil" (page 33).
The Book of Mormon says, "And behold, it is he who is the author of all sin. And behold, he doth carry on his works of darkness and secret murder" (Helaman 6:30).
"Because Satan had introduced evil into the world, Adam and Eve and their children were separated from God both physically and spiritually" (Gospel Principles, page 33).
Do you believe that evil = disobedience = sin?
From the scriptures we know that Adam and Eve died spiritually and physically as a result of their transgression. They were separated from God before they ever brought children into the world. Their children were subsequently born in a spiritually dead (separated from God) condition (Alma 42:9).
While The Church of Jesus Christ of Latter-day Saints teaches that great blessings resulted from Adam and Eve's disobedience (see Gospel Principles, page 33), she does not explicitly teach today that Adam and Eve sinned in the Garden of Eden. A former president of the LDS Church taught, "He [mankind] dies in consequence of the sin of Adam” (Teachings of Presidents of the Church - Joseph F. Smith, Chapter 10, p. 89).
The Bible says, "For as by one man's disobedience many were made sinners, so by the obedience of one shall many be made righteous" (Romans 5:19).
Latter-day Saints do not believe this Biblical teaching. They believe instead that one becomes a sinner after they sin first.
The LDS Church also teaches that disobedience to God resulted in Adam and Eve gaining the ability to have children and joy. "As God had promised, they became mortal. They were able to have children" (Gospel Principles, page 33; see also 2 Nephi 2:22-23; Moses 5:10-11).