Sound Understanding

15And to bring about his eternal purposes in the end of man, after he had created our first parents, and the beasts of the field and the fowls of the air, and in fine, all things which are created, it must needs be that there was an opposition; even the forbidden fruit in opposition to the tree of life; the one being sweet and the other bitter.

“his eternal purposes”
God's work and glory is to bring to pass the immortality and eternal life of man.
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“it must needs be that there was an opposition”
Agency to choose requires opposition; without a law that could be transgressed, the consequences of the Fall could not have been justly imposed.

16Wherefore, the Lord God gave unto man that he should act for himself. Wherefore, man could not act for himself save it should be that he was enticed by the one or the other.

“enticed by the one or the other”
Agency is a gift from God, but it required an opposing influence.

17And I, Lehi, according to the things which I have read, must needs suppose that an angel of God, according to that which is written, had fallen from heaven; wherefore, he became a devil, having sought that which was evil before God.

18And because he had fallen from heaven, and had become miserable forever, he sought also the misery of all mankind. Wherefore, he said unto Eve, yea, even that old serpent, who is the devil, who is the father of all lies, wherefore he said: Partake of the forbidden fruit, and ye shall not die, but ye shall be as God, knowing good and evil.

19And after Adam and Eve had partaken of the forbidden fruit they were driven out of the garden of Eden, to till the earth.

“driven out of the garden of Eden”
The physical removal from the presence of God is spiritual death, which the Fall brought upon all mankind.
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20And they have brought forth children; yea, even the family of all the earth.

21And the days of the children of men were prolonged, according to the will of God, that they might repent while in the flesh; wherefore, their state became a state of probation, and their time was lengthened, according to the commandments which the Lord God gave unto the children of men. For he gave commandment that all men must repent; for he showed unto all men that they were lost, because of the transgression of their parents.

22And now, behold, if Adam had not transgressed he would not have fallen, but he would have remained in the garden of Eden. And all things which were created must have remained in the same state in which they were after they were created; and they must have remained forever, and had no end.

“if Adam had not transgressed”
The act that produced the Fall is called a transgression, not a sin, echoing the second article of faith. See the teaching of Dallin H. Oaks on the careful choice of that word.
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23And they would have had no children; wherefore they would have remained in a state of innocence, having no joy, for they knew no misery; doing no good, for they knew no sin.

24But behold, all things have been done in the wisdom of him who knoweth all things.

25Adam fell that men might be; and men are, that they might have joy.

“Adam fell that men might be; and men are, that they might have joy”
Adam's fall initiated the mortal, probationary state for all mankind, and joy comes through the agency made possible by the Redemption.

26And the Messiah cometh in the fulness of time, that he may redeem the children of men from the fall. And because that they are redeemed from the fall they have become free forever, knowing good from evil; to act for themselves and not to be acted upon, save it be by the punishment of the law at the great and last day, according to the commandments which God hath given.

“the Messiah cometh ... that he may redeem the children of men from the fall”
This introduces the causal relationship between the Fall and the Atonement.
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2 Nephi 2:15-26