The Biblical narrative of the fall of humanity, original sin, the first murder, etc. present some questions which have puzzled Bible readers for centuries. Some of the most often questions I receive are “Where did Cain get his wife?” and “Who were the nefilim?”
Answering these questions requires an understanding of Hebrew grammar and syntax- especially of narrative structure. The first thing to understand is that the Bible is not always written in a chronological sequence; this means that when a narrative is read, it must not demand that what came first in the narrative was always the first in the narrative sequence.
An easy explanation of this can be drawn from the first two verses of the Bible, “In the beginning, God created the heavens and the earth. And the earth was formless and void and darkness was upon the face of the deep and the spirit of God was hovering over the face of the waters.” When one reads this in English, or many other translations, one might think the first two verses are in the same narrative sequence; this, however, is incorrect.
In Hebrew, the conjunction and [Waw וְ ] has several functions; the conjunction can be a temporal modifier signaling a transition from an imperfect to perfect- this is with verbs. The conjunctions can signal a continuation of a narrative sequence- the most common use of the conjunction. The conjunction can also be used as a disjunctive- a break or pause in a narrative sequence. In this case, the conjunction signifies a parenthetical narrative in the narrative sequence or a break in the prior narrative sequence and the beginning of a new narrative.
As a disjunctive, the Hebrew and [וְ -Waw] is prefixed to a non-verb in the beginning of its clause; this is what occurred in the second verse above- “And the Earth was…” This construction indicated a disjunction from the preceding verse. The creation narrative, then, actually began with the second verse and not with the first verse. This use of the disjunctive Waw [and] is employed in the narratives which followed the creation narrative: Genesis 3:1 “And the serpent was…;” Genesis 4:1 “And the man knew…;” Genesis 4:4 “And Abel brought…” In each of these cases, the use of the disjunctive Waw [and] indicated a break from the previous narrative sequence.
Genesis 3:1 indicated the break in the creation narrative sequence; it began a new narrative sequence from the point of view of the initial sin of Eve. It was not a chronological sequence but could have been up to 130 years after the events of the creation of man. Later, in Genesis, it is explained that Adam was 130 years old when Seth was born- after their expulsion from the Garden of Eden. In the midst of this narrative, Genesis 4:1 presented a new, parenthetical narrative sequence. This, too, is irrespective of time- when this took place was never clearly mentioned. The use of the disjunctive Waw here indicated that before the temptation of Eve, the children were born; this is the natural understanding of the Hebrew Text when the first commandment God ever gave Adam and Eve was to be “fruitful and multiply.” That they could have spent as much as 130 years in the Garden of Eden without procreation meant that they spent up to 130 years disobeying God’s first commandment. This use of the disjunctive clearly demonstrated that the children- who were twins- were born in the Garden and were not expelled with Adam and Eve 
The expulsion of Adam and Eve was not a simple command to leave the Garden but was a rather forceful and possibly a violent act. Neither of the two wished to leave the Garden of God. This is indicated by the use of the two verbs sent [שָׁלַח- shalah] and drove [גָרַשׁ- garash] in 3:23 and 24. The verb sent indicated that they were requested to leave and that they were, possibly, accompanied to the border of the Garden. The verb drove indicated that this was not a consensual leaving on the part of Adam and Eve, but they were compelled to go; this is made even more clear by the fact that the way back was guarded by cherubs wielding flaming swords.
The narrative of the birth of Cain and Abel is an interruption in the sequence of the expulsion of Adam and Eve; it explained an event which occurred prior to Adam and Eve being expelled- the birth of the twins. This narrative is immediately interrupted by the last of the disjunctive Waw clauses- Genesis 4:4 “And Abel brought…”
The new narrative began from the point of view of Cain and Abel who were adults and still living in the Garden after the expulsion of their parents. The disjunctive interrupted the narrative from the clause, “And in process of time it came to pass.” The phrase “process of time” is [מִקֵּץ יָמִים -miqqets yamim] in Hebrew and meant the end of an age. Sometimes, it was translated as “end of days.” To what end of the age is being referenced is clear- the end of the age of innocence. At the conclusion of this narrative sequence, there would be none left who had not sinned against God.
To know, for sure, that Cain was born in a natural state of perfection, it is simple to point out that the Erbsünde [inherited sin] could not have applied to Cain. This is made clear by the statement of God, “If you do well, will you not be accepted? And if you do not do well, sin is crouching at the door. Its desire is for you, but you must rule over it” (Genesis 4:7). The final clause is written in the imperfect in the Hebrew [וְאַתָּה תִּמְשָׁל-בּוֹ -weAttah timshol-bo; yet you, you rule over it]. This is not permissive nor an offer of advice, but a statement of fact; at this point, Cain ruled over sin- it had no power over him just like it had no power over his parents. After this, God said this to no other person.
A comparison of the two narratives- that of the expulsion of Adam and Eve and the expulsion of Cain- will explain to where they were all expelled. Adam and Eve were expelled, rather forcefully, to the East of the Garden of Eden (Genesis 3;24). It was outside the Garden of Eden which came to be known as the Land of Nod [נֹוד -nod meaning wandering]. As opposed to his parents, Cain willingly left from the presence of God- which was in the Garden (Genesis 3:8). This is demonstrated by the lack of the two verbs sent and drove; rather it stated that Cain “went out.” The direction of travel, as in the case of his parents, was to the East- to the Land of Wandering (Genesis 4:16).
Cain left from Eden and went to the same land where his parents had been sent; this raises the question, “How long were Adam and Eve in the Land of Nod?” We already know that Adam was 130 years at the birth of Seth; before he was born, Adam had already given birth to many other children- including women. It is from the women born to Adam that Cain took his wife; this marriage and mixture led to the events of Genesis 6 and the deluge. It was Cain’s lineage who invented weapons of war (Genesis 4:22), instruments of pleasure (Genesis 4:21), and were the first to take possessions (Genesis 4:20); his children followed his example as explained from the confession of Lamech- who had also killed many people. Adam’s line- through Seth- were sorry for their sins and tried to amend their ways and walk with God; Cain’s line, however, was unrepentant and spread violence and corruption over the face of the Earth. The Godly line was called the sons of God while the sinful line was called the sons of Adam [men].
The events of Genesis 4 and 5 led to the events of chapter 6. There have been volumes of wasted ink on this subject- most of which ignore the contextual backdrop to the chapter and invent fairy-tales of some sort of inbreeding between humans and angels.
Most of the speculation surrounds verses 1-4 with a strong emphasis on verse 4 in particular.
Gen 6:1 Now when humankind [הָאָדָם -haAdam; the man] began to multiply on the face of the ground and daughters were born to them,
Gen 6:2 and the sons of God [benei haElohim] saw that the daughters of men were good and they took for themselves wives, any they chose.
Gen 6:3 Then the Lord said, “My Spirit will not quarrel  with humankind [baAdam- with the man] forever, since they are flesh. So their days will be 120 years.”
Gen 6:4 The nephilim [הַנְּפִלִים] were on the earth in those days, and also afterward when the sons of God came to the daughters of men and gave birth to them. Those were the mighty men of old, men of renown.
Adam [אָדָם] is the name that God called humanity- both female and male- in the day He created them.
Gen 5:1 This is the Book of the Genealogies of Adam [אָדָם]: When God created Adam, in the likeness of God He made him.
Gen 5:2 Male and female He created them, and He blessed them and called their name “Adam” when He created them.
The humans which followed God and sought to amend their ways were then, and thereafter, referred to as sons of God [בְּנֵי־הָאֱלֹהִים -benei haElohim]:
Ye are the children of the LORD your God Deuteronomy 14:1
it shall be said unto them, Ye are the sons of the living God. Hosea 1:10
The idea behind the phrase benei haElohim [בְּנֵי־הָאֱלֹהִים – sons of God] is not one of absolute sinlessness, nor of being always on the side of right; the idea is and always has been one of contriteness and a readiness to do what is right in the face of bad decisions. After the birth of Seth, in the days of his son Enosh [mortal man], the line of Adam began to call upon the name of the Lord (Genesis 4:26). In Genesis 5:22, it stated that Enoch [Hanokh- dedicated] walked with God. The verb used for “walk” is hit-halek [הִתְהַלֵּךְ] in Hebrew; this is both a reciprocal verb stem as well as a reflexive and meant that one did this for or to themselves as well as in conjunction with another. The idea is that Enoch sought to amend his ways, by walking in the path of God, and God blessed him for it.
It is the same idea behind King David being called a “man after God’s heart” even though he committed murder and adultery; after the crimes were committed, David was genuinely sorry and sought to reform his ways and make restitution for his bad decisions. Repentance [תְּשׁוּבָה -teshuvah] is the key:
“Now when all these things come upon you—the blessing and the curse that I have set before you—and you take them to heart in all the nations where the Lord your God has banished you, and you return [וְשַׁבְתָּ -weshavta] to the Lord your God and listen to His voice according to all that I am commanding you today—you and your children—with all your heart and with all your soul, then the Lord your God will bring you back from captivity and have compassion on you, and He will return and gather you from all the peoples where the Lord your God has scattered you. Even if your outcasts are at the ends of the heavens, from there the Lord your God will gather you, and from there He will bring you.
Deuteronomy 30:1-4 [see also Deuteronomy 4:24-31]
“Do I delight at all in the death of the wicked?” It is a declaration of Adonai. “Rather, should he not return [בְּשׁוּבֹו -beshuvo] from his ways, and live? Ezekiel 18:23
In contrast to the godly line of Adam, the line of Cain was not sorry and sought no reform; on the other hand, they continued in their murderous ways and developed the art of pleasure and war. This line was called the nephilim. Nefilim is a plural passive participle from the verb NaFaL- to fall; the passive participle is used, more often than not, as an adjective in Hebrew. The nephilim are those people who were in a fallen state. The choice of word used in this narrative echoed back to the fall of Cain himself:
Cain became very angry, and his countenance fell [וַיִּפְּלוּ -wayYippelu]. Then the Lord said to Cain, “Why are you angry? And why has your countenance fallen [נָפְלוּ -noflu]? Genesis 4:5-6
The idea behind the verb NaFaL [נָפַל]- when applied to humans- can have several meanings; the most common meaning, when applied to humans, is that of death, or one fallen in battle. The nephilim in Genesis 6:4, however, was alive and not dead; the meaning in this case, then, is that of a person from one camp defecting to the people of another camp- they changed sides. Consider the following verse:
Then the remnant of the people who were left in the city—the deserters [הַנֹּפְלִים -haNofelim] who had defected [נָפְלוּ -noflu] to the Babylonian king and the rest of the populace—Nebuzaradan captain of the guard exiled them. 2 Kings 25:11
The nephilim in Genesis 6:4, then, were those who were formerly godly but fell to the wicked ways of the sons of Cain. This is easily proven when considering the verse:
The nephilim were on the earth in those days, and also afterward, whenever the sons of God came to the daughters of men and gave birth to them. Those were the mighty men of old, men of renown.
The last clause of this verse, in Hebrew, is actually a bit different than what is commonly translated:
הֵמָּה הַגִּבֹּרִים אֲשֶׁר מֵעוֹלָם אַנְשֵׁי הַשֵּׁם
These were the gibborim [גִּבֹּרִים] which were from ancient times [מֵעוֹלָם -me’Olam] men of God [אַנְשֵׁי הַשֵּׁם -anshe HaShem].
Gibborim [גִּבֹּרִים], in Hebrew, represented men of war, or men of great valor; David’s warriors were called gibborim (2Samuel 23:8)- so too was Nimrod called a gibbor [גִּבֹּר] (Genesis 10:8). The line of Cain was responsible for spreading violence in the Earth and the mixture of the godly line of Adam ensured that their wicked ways would dominate the world. These wicked men spread violence and war; in the end, God saw the world destroyed from these people:
God saw the earth, and behold it was destroyed because all flesh had destroyed their way upon the earth. Genesis 6:12
Only Noah was a righteous man and always sought to walk with God; for this reason, God spared the life of Noah and his family.
Noah was a righteous man. He was blameless among his generation. Noah continually walked with God. Genesis 6:9
For you only do I perceive as righteous before Me in this generation. Genesis 7:1
This narrative dealt with the consequences of the fall of humanity, the faith and righteous acts of the men of God, and their deliverance and reward for their faithfulness to God’s law. It never had anything to do with fallen angels fathering hybrid children with humans and creating a race of giants or supermen. In fact, angels are not mentioned in this narrative nor were they the objects of divine wrath.
 Rashi, the medieval Rabbi and Torah commentator Shlomo Yitzchaki, explained concerning this verse: והאדם ידע AND THE MAN KNEW already before the events related above look place — before he sinned and was driven out of the Garden of Eden. So, also, the conception and birth of Cain took place before this. Had it been written, וידע אדם it would imply that after he was driven out children were born to him (Genesis Rabbah 22:2).
 My Spirit will not quarrel with humankind forever, since they are flesh. The verb used I translated as quarrel is יָדוֹן- yadon from the verb root [דין] D-Y-N; this word is used in reference to quarreling or disputing over a case of law or a matter of controversy- similar to trying a case in court. Consider the following:
Open your mouth, judge righteously, plead [וְדִין -wedin] the cause of the poor and needy. Proverb 31:9
All the people throughout all the tribes of Israel were at strife [נָדוֹן -nadon] saying, “The king delivered us from the hand of our enemies and he saved us from the hand of the Philistines. Yet now he had to flee from the land because of Absalom, 2Smauel 19:10