Moses 4-6; Genesis 2-3.
The Fall of Man is an integral part of Heavenly Father's plan. Elder Bruce R. McConkie explained that our salvation is contingent upon three pillars of eternity: the Creation, the Fall, and the Atonement. Even though our Father knew that the Fall was a necessary part in our progression to return to Him and to become like Him, it had to be a mortal decision to partake of the fruit of the Tree of Knowledge of Good and Evil.
So it appeared that God gave two conflicting commandments: first, for Adam and Eve to be fruitful and multiply and replenish the earth; second, that they could eat of any fruit of any tree in the Garden of Eden, but that they must not partake of the fruit of the Tree of Knowledge of Good and Evil. Nevertheless, the Lord told them, it is given unto thee, but in the day that you of it, you shall surely die. So there would be a consequence if they took of the fruit. They would be cut off from the Lord's presence, spiritual death, and they would not live past 1,000 years (which is one day on Kolob), physical death.
Satan appeared to Adam through a serpent and tempted him to eat the fruit. But Father Adam, true to the integrity of his immortal soul would not partake, but he would keep the Father's commandments with exactness and honor. So Satan then turned to Eve, and he beguiled her. Her explained that she would not die, which she certainly would because God had spoken it, and that her eyes would be opened and she would become as the gods knowing good and evil. He said that God also gained His knowledge that way. She questioned, "Is there no other way." Satan answered, "There is no other way." Eve believed Satan's half-truths and partook of the fruit. When Eve explained to Adam that he must partake in order for them to remain together, Adam said, "Eve, I see that this must be. I will partake that man may be." So their eyes were opened, they knew they were naked; it was necessary for them to be cast out of the Garden so that they could not partake of the Tree of Life and live forever in their sins.
So man was given a probationary time: a time to repent and to prepare to meet God. Adam and Eve became mortal and were given the powers of procreation. They were able to have children. Because of agency, they were capable of now choosing between good and evil and they could recognize good and evil. They could experience both joy and sorrow, and they could feel the joy of their redemption. Because of the Atonement, they could obtain Eternal Life. Angels were sent to teach Adam and Eve the gospel. They were both baptized and given the Gift of the Holy Ghost. This also testified to the truth of the gospel.
If we recognize that Adam fell, then we can recognize the need for a Savior: One who could conquer death for all mankind and One who could bring us back into the presence of our Heavenly Father. Adam and Eve were commanded to offer blood sacrifice. This sacrifice was the firstling of the flock, pure and without blemish. This sacrifice represented the Son of God who would be offered for all mankind, both a sacrifice for our sins and a way for us to overcome both physical and spiritual death.
The reasons for the Fall and the Atonement are recorded beautifully in 2 Nephi 2. Eve said it best, in Moses 5:11: "Were it not for our transgression we never should have had seed, and never should have known good and evil, and the joy of our redemption, and the eternal life which God giveth unto all the obedient."
Thanks be to Adam and Eve for the important step they took. Because of them, we have all had the opportunity to come to this earth and be given bodies and gain experience. Thanks be to Jesus, and Heavenly Father for sending Him, who has prepared an excellent way for our deliverance from death and sin and for our return to Him.